diff --git a/ChangeLog.d/fix-pk-parse-key-error-code.txt b/ChangeLog.d/fix-pk-parse-key-error-code.txt
new file mode 100644
index 0000000000..3aa330b1a5
--- /dev/null
+++ b/ChangeLog.d/fix-pk-parse-key-error-code.txt
@@ -0,0 +1,2 @@
+Bugfix
+   * Fix an incorrect error code when parsing a PKCS#8 private key.
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 0ea1d065d1..7936f6b786 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -30,6 +30,11 @@
 
 #include <stddef.h>
 
+#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
+    !defined(inline) && !defined(__cplusplus)
+#define inline __inline
+#endif
+
 /**
  * Error code layout.
  *
@@ -117,6 +122,54 @@ extern "C" {
 #define MBEDTLS_ERR_PLATFORM_HW_ACCEL_FAILED     -0x0070 /**< Hardware accelerator failed */
 #define MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED -0x0072 /**< The requested feature is not supported by the platform */
 
+/**
+ * \brief Combines a high-level and low-level error code together.
+ *
+ *        Wrapper macro for mbedtls_error_add(). See that function for
+ *        more details.
+ */
+#define MBEDTLS_ERROR_ADD( high, low ) \
+        mbedtls_error_add( high, low, __FILE__, __LINE__ )
+
+#if defined(MBEDTLS_TEST_HOOKS)
+/**
+ * \brief Testing hook called before adding/combining two error codes together.
+ *        Only used when invasive testing is enabled via MBEDTLS_TEST_HOOKS.
+ */
+extern void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
+#endif
+
+/**
+ * \brief Combines a high-level and low-level error code together.
+ *
+ *        This function can be called directly however it is usually
+ *        called via the #MBEDTLS_ERROR_ADD macro.
+ *
+ *        While a value of zero is not a negative error code, it is still an
+ *        error code (that denotes success) and can be combined with both a
+ *        negative error code or another value of zero.
+ *
+ * \note  When invasive testing is enabled via #MBEDTLS_TEST_HOOKS, also try to
+ *        call \link mbedtls_test_hook_error_add \endlink.
+ *
+ * \param high      high-level error code. See error.h for more details.
+ * \param low       low-level error code. See error.h for more details.
+ * \param file      file where this error code addition occured.
+ * \param line      line where this error code addition occured.
+ */
+static inline int mbedtls_error_add( int high, int low,
+                                     const char *file, int line )
+{
+#if defined(MBEDTLS_TEST_HOOKS)
+    if( *mbedtls_test_hook_error_add != NULL )
+        ( *mbedtls_test_hook_error_add )( high, low, file, line );
+#endif
+    (void)file;
+    (void)line;
+
+    return( high + low );
+}
+
 /**
  * \brief Translate a mbed TLS error code into a string representation,
  *        Result is truncated if necessary and always includes a terminating
diff --git a/library/dhm.c b/library/dhm.c
index f796812310..9758af7872 100644
--- a/library/dhm.c
+++ b/library/dhm.c
@@ -79,7 +79,7 @@ static int dhm_read_bignum( mbedtls_mpi *X,
         return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );
 
     if( ( ret = mbedtls_mpi_read_binary( X, *p, n ) ) != 0 )
-        return( MBEDTLS_ERR_DHM_READ_PARAMS_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_READ_PARAMS_FAILED, ret ) );
 
     (*p) += n;
 
@@ -222,7 +222,7 @@ int mbedtls_dhm_make_params( mbedtls_dhm_context *ctx, int x_size,
 cleanup:
 
     if( ret != 0 )
-        return( MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED, ret ) );
 
     return( 0 );
 }
@@ -242,7 +242,7 @@ int mbedtls_dhm_set_group( mbedtls_dhm_context *ctx,
     if( ( ret = mbedtls_mpi_copy( &ctx->P, P ) ) != 0 ||
         ( ret = mbedtls_mpi_copy( &ctx->G, G ) ) != 0 )
     {
-        return( MBEDTLS_ERR_DHM_SET_GROUP_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_SET_GROUP_FAILED, ret ) );
     }
 
     ctx->len = mbedtls_mpi_size( &ctx->P );
@@ -263,7 +263,7 @@ int mbedtls_dhm_read_public( mbedtls_dhm_context *ctx,
         return( MBEDTLS_ERR_DHM_BAD_INPUT_DATA );
 
     if( ( ret = mbedtls_mpi_read_binary( &ctx->GY, input, ilen ) ) != 0 )
-        return( MBEDTLS_ERR_DHM_READ_PUBLIC_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_READ_PUBLIC_FAILED, ret ) );
 
     return( 0 );
 }
@@ -313,7 +313,7 @@ int mbedtls_dhm_make_public( mbedtls_dhm_context *ctx, int x_size,
 cleanup:
 
     if( ret != 0 )
-        return( MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED, ret ) );
 
     return( 0 );
 }
@@ -462,7 +462,7 @@ cleanup:
     mbedtls_mpi_free( &GYb );
 
     if( ret != 0 )
-        return( MBEDTLS_ERR_DHM_CALC_SECRET_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_CALC_SECRET_FAILED, ret ) );
 
     return( 0 );
 }
@@ -544,7 +544,7 @@ int mbedtls_dhm_parse_dhm( mbedtls_dhm_context *dhm, const unsigned char *dhmin,
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
-        ret = MBEDTLS_ERR_DHM_INVALID_FORMAT + ret;
+        ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_INVALID_FORMAT, ret );
         goto exit;
     }
 
@@ -553,7 +553,7 @@ int mbedtls_dhm_parse_dhm( mbedtls_dhm_context *dhm, const unsigned char *dhmin,
     if( ( ret = mbedtls_asn1_get_mpi( &p, end, &dhm->P  ) ) != 0 ||
         ( ret = mbedtls_asn1_get_mpi( &p, end, &dhm->G ) ) != 0 )
     {
-        ret = MBEDTLS_ERR_DHM_INVALID_FORMAT + ret;
+        ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_INVALID_FORMAT, ret );
         goto exit;
     }
 
@@ -567,13 +567,13 @@ int mbedtls_dhm_parse_dhm( mbedtls_dhm_context *dhm, const unsigned char *dhmin,
         mbedtls_mpi_free( &rec );
         if ( ret != 0 )
         {
-            ret = MBEDTLS_ERR_DHM_INVALID_FORMAT + ret;
+            ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_INVALID_FORMAT, ret );
             goto exit;
         }
         if ( p != end )
         {
-            ret = MBEDTLS_ERR_DHM_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
+            ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_DHM_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
             goto exit;
         }
     }
diff --git a/library/ecdsa.c b/library/ecdsa.c
index 7dc8708a3f..7f259e1055 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -870,8 +870,8 @@ int mbedtls_ecdsa_read_signature_restartable( mbedtls_ecdsa_context *ctx,
 
     if( p + len != end )
     {
-        ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA +
-              MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
+        ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
+              MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
         goto cleanup;
     }
 
diff --git a/library/error.c b/library/error.c
index f489be6dc1..a176deb8c5 100644
--- a/library/error.c
+++ b/library/error.c
@@ -858,4 +858,8 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
 
 #endif /* MBEDTLS_ERROR_C */
 
+#if defined(MBEDTLS_TEST_HOOKS)
+void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
+#endif
+
 #endif /* MBEDTLS_ERROR_C || MBEDTLS_ERROR_STRERROR_DUMMY */
diff --git a/library/pem.c b/library/pem.c
index 969d492e3b..fcfde94799 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -343,7 +343,7 @@ int mbedtls_pem_read_buffer( mbedtls_pem_context *ctx, const char *header, const
     ret = mbedtls_base64_decode( NULL, 0, &len, s1, s2 - s1 );
 
     if( ret == MBEDTLS_ERR_BASE64_INVALID_CHARACTER )
-        return( MBEDTLS_ERR_PEM_INVALID_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PEM_INVALID_DATA, ret ) );
 
     if( ( buf = mbedtls_calloc( 1, len ) ) == NULL )
         return( MBEDTLS_ERR_PEM_ALLOC_FAILED );
@@ -352,7 +352,7 @@ int mbedtls_pem_read_buffer( mbedtls_pem_context *ctx, const char *header, const
     {
         mbedtls_platform_zeroize( buf, len );
         mbedtls_free( buf );
-        return( MBEDTLS_ERR_PEM_INVALID_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PEM_INVALID_DATA, ret ) );
     }
 
     if( enc != 0 )
diff --git a/library/pkcs12.c b/library/pkcs12.c
index 4bdeb6835e..9823d963c3 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -60,21 +60,21 @@ static int pkcs12_parse_pbe_params( mbedtls_asn1_buf *params,
      *
      */
     if( params->tag != ( MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) )
-        return( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &salt->len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT, ret ) );
 
     salt->p = *p;
     *p += salt->len;
 
     if( ( ret = mbedtls_asn1_get_int( p, end, iterations ) ) != 0 )
-        return( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT, ret ) );
 
     if( *p != end )
-        return( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS12_PBE_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
diff --git a/library/pkcs5.c b/library/pkcs5.c
index e9e743fa90..2b014d91c8 100644
--- a/library/pkcs5.c
+++ b/library/pkcs5.c
@@ -60,8 +60,8 @@ static int pkcs5_parse_pbkdf2_params( const mbedtls_asn1_buf *params,
     const unsigned char *end = params->p + params->len;
 
     if( params->tag != ( MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
     /*
      *  PBKDF2-params ::= SEQUENCE {
      *    salt              OCTET STRING,
@@ -73,13 +73,13 @@ static int pkcs5_parse_pbkdf2_params( const mbedtls_asn1_buf *params,
      */
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &salt->len,
                                       MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT, ret ) );
 
     salt->p = p;
     p += salt->len;
 
     if( ( ret = mbedtls_asn1_get_int( &p, end, iterations ) ) != 0 )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT, ret ) );
 
     if( p == end )
         return( 0 );
@@ -87,21 +87,21 @@ static int pkcs5_parse_pbkdf2_params( const mbedtls_asn1_buf *params,
     if( ( ret = mbedtls_asn1_get_int( &p, end, keylen ) ) != 0 )
     {
         if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
-            return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT, ret ) );
     }
 
     if( p == end )
         return( 0 );
 
     if( ( ret = mbedtls_asn1_get_alg_null( &p, end, &prf_alg_oid ) ) != 0 )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT, ret ) );
 
     if( mbedtls_oid_get_md_hmac( &prf_alg_oid, md_type ) != 0 )
         return( MBEDTLS_ERR_PKCS5_FEATURE_UNAVAILABLE );
 
     if( p != end )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -134,12 +134,12 @@ int mbedtls_pkcs5_pbes2( const mbedtls_asn1_buf *pbe_params, int mode,
      *  }
      */
     if( pbe_params->tag != ( MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     if( ( ret = mbedtls_asn1_get_alg( &p, end, &kdf_alg_oid,
                                       &kdf_alg_params ) ) != 0 )
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT, ret ) );
 
     // Only PBKDF2 supported at the moment
     //
@@ -160,7 +160,7 @@ int mbedtls_pkcs5_pbes2( const mbedtls_asn1_buf *pbe_params, int mode,
     if( ( ret = mbedtls_asn1_get_alg( &p, end, &enc_scheme_oid,
                               &enc_scheme_params ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PKCS5_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PKCS5_INVALID_FORMAT, ret ) );
     }
 
     if( mbedtls_oid_get_cipher_alg( &enc_scheme_oid, &cipher_alg ) != 0 )
diff --git a/library/pkparse.c b/library/pkparse.c
index 0590f2b050..3222ca20fb 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -186,8 +186,8 @@ static int pk_get_ecparams( unsigned char **p, const unsigned char *end,
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     if ( end - *p < 1 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     /* Tag may be either OID or SEQUENCE */
     params->tag = **p;
@@ -197,21 +197,21 @@ static int pk_get_ecparams( unsigned char **p, const unsigned char *end,
 #endif
             )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
     }
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &params->len, params->tag ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     params->p = *p;
     *p += params->len;
 
     if( *p != end )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -247,7 +247,7 @@ static int pk_group_from_specified( const mbedtls_asn1_buf *params, mbedtls_ecp_
 
     /* SpecifiedECDomainVersion ::= INTEGER { 1, 2, 3 } */
     if( ( ret = mbedtls_asn1_get_int( &p, end, &ver ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( ver < 1 || ver > 3 )
         return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT );
@@ -285,13 +285,13 @@ static int pk_group_from_specified( const mbedtls_asn1_buf *params, mbedtls_ecp_
 
     /* Prime-p ::= INTEGER -- Field of size p. */
     if( ( ret = mbedtls_asn1_get_mpi( &p, end_field, &grp->P ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     grp->pbits = mbedtls_mpi_bitlen( &grp->P );
 
     if( p != end_field )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     /*
      * Curve ::= SEQUENCE {
@@ -315,7 +315,7 @@ static int pk_group_from_specified( const mbedtls_asn1_buf *params, mbedtls_ecp_
     if( ( ret = mbedtls_asn1_get_tag( &p, end_curve, &len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 ||
         ( ret = mbedtls_mpi_read_binary( &grp->A, p, len ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     p += len;
@@ -323,7 +323,7 @@ static int pk_group_from_specified( const mbedtls_asn1_buf *params, mbedtls_ecp_
     if( ( ret = mbedtls_asn1_get_tag( &p, end_curve, &len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 ||
         ( ret = mbedtls_mpi_read_binary( &grp->B, p, len ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     p += len;
@@ -333,14 +333,14 @@ static int pk_group_from_specified( const mbedtls_asn1_buf *params, mbedtls_ecp_
         p += len;
 
     if( p != end_curve )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     /*
      * ECPoint ::= OCTET STRING
      */
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( ( ret = mbedtls_ecp_point_read_binary( grp, &grp->G,
                                       ( const unsigned char *) p, len ) ) != 0 )
@@ -366,7 +366,7 @@ static int pk_group_from_specified( const mbedtls_asn1_buf *params, mbedtls_ecp_
      * order INTEGER
      */
     if( ( ret = mbedtls_asn1_get_mpi( &p, end, &grp->N ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     grp->nbits = mbedtls_mpi_bitlen( &grp->N );
 
@@ -528,15 +528,15 @@ static int pk_get_rsapubkey( unsigned char **p,
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY, ret ) );
 
     if( *p + len != end )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     /* Import N */
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len, MBEDTLS_ASN1_INTEGER ) ) != 0 )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY, ret ) );
 
     if( ( ret = mbedtls_rsa_import_raw( rsa, *p, len, NULL, 0, NULL, 0,
                                         NULL, 0, NULL, 0 ) ) != 0 )
@@ -546,7 +546,7 @@ static int pk_get_rsapubkey( unsigned char **p,
 
     /* Import E */
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len, MBEDTLS_ASN1_INTEGER ) ) != 0 )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY, ret ) );
 
     if( ( ret = mbedtls_rsa_import_raw( rsa, NULL, 0, NULL, 0, NULL, 0,
                                         NULL, 0, *p, len ) ) != 0 )
@@ -561,8 +561,8 @@ static int pk_get_rsapubkey( unsigned char **p,
     }
 
     if( *p != end )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -584,7 +584,7 @@ static int pk_get_pk_alg( unsigned char **p,
     memset( params, 0, sizeof(mbedtls_asn1_buf) );
 
     if( ( ret = mbedtls_asn1_get_alg( p, end, &alg_oid, params ) ) != 0 )
-        return( MBEDTLS_ERR_PK_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_ALG, ret ) );
 
     if( mbedtls_oid_get_pk_alg( &alg_oid, pk_alg ) != 0 )
         return( MBEDTLS_ERR_PK_UNKNOWN_PK_ALG );
@@ -624,7 +624,7 @@ int mbedtls_pk_parse_subpubkey( unsigned char **p, const unsigned char *end,
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                     MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     end = *p + len;
@@ -633,11 +633,11 @@ int mbedtls_pk_parse_subpubkey( unsigned char **p, const unsigned char *end,
         return( ret );
 
     if( ( ret = mbedtls_asn1_get_bitstring_null( p, end, &len ) ) != 0 )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY, ret ) );
 
     if( *p + len != end )
-        return( MBEDTLS_ERR_PK_INVALID_PUBKEY +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     if( ( pk_info = mbedtls_pk_info_from_type( pk_alg ) ) == NULL )
         return( MBEDTLS_ERR_PK_UNKNOWN_PK_ALG );
@@ -662,8 +662,8 @@ int mbedtls_pk_parse_subpubkey( unsigned char **p, const unsigned char *end,
         ret = MBEDTLS_ERR_PK_UNKNOWN_PK_ALG;
 
     if( ret == 0 && *p != end )
-        ret = MBEDTLS_ERR_PK_INVALID_PUBKEY +
-              MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
+        ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY,
+              MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
 
     if( ret != 0 )
         mbedtls_pk_free( pk );
@@ -734,14 +734,14 @@ static int pk_parse_key_pkcs1_der( mbedtls_rsa_context *rsa,
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
 
     if( ( ret = mbedtls_asn1_get_int( &p, end, &version ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     if( version != 0 )
@@ -831,8 +831,8 @@ static int pk_parse_key_pkcs1_der( mbedtls_rsa_context *rsa,
 
     if( p != end )
     {
-        ret = MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-              MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ;
+        ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+              MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
     }
 
 cleanup:
@@ -843,7 +843,7 @@ cleanup:
     {
         /* Wrap error code if it's coming from a lower level */
         if( ( ret & 0xff80 ) == 0 )
-            ret = MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret;
+            ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret );
         else
             ret = MBEDTLS_ERR_PK_KEY_INVALID_FORMAT;
 
@@ -883,24 +883,24 @@ static int pk_parse_key_sec1_der( mbedtls_ecp_keypair *eck,
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
 
     if( ( ret = mbedtls_asn1_get_int( &p, end, &version ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( version != 1 )
         return( MBEDTLS_ERR_PK_KEY_INVALID_VERSION );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( ( ret = mbedtls_mpi_read_binary( &eck->d, p, len ) ) != 0 )
     {
         mbedtls_ecp_keypair_free( eck );
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     p += len;
@@ -924,7 +924,7 @@ static int pk_parse_key_sec1_der( mbedtls_ecp_keypair *eck,
         else if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
         {
             mbedtls_ecp_keypair_free( eck );
-            return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
         }
     }
 
@@ -940,11 +940,11 @@ static int pk_parse_key_sec1_der( mbedtls_ecp_keypair *eck,
             end2 = p + len;
 
             if( ( ret = mbedtls_asn1_get_bitstring_null( &p, end2, &len ) ) != 0 )
-                return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
             if( p + len != end2 )
-                return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                        MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                        MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
             if( ( ret = pk_get_ecpubkey( &p, end2, eck ) ) == 0 )
                 pubkey_done = 1;
@@ -961,7 +961,7 @@ static int pk_parse_key_sec1_der( mbedtls_ecp_keypair *eck,
         else if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
         {
             mbedtls_ecp_keypair_free( eck );
-            return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
         }
     }
 
@@ -970,7 +970,7 @@ static int pk_parse_key_sec1_der( mbedtls_ecp_keypair *eck,
                                                       NULL, NULL ) ) != 0 )
     {
         mbedtls_ecp_keypair_free( eck );
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     if( ( ret = mbedtls_ecp_check_privkey( &eck->grp, &eck->d ) ) != 0 )
@@ -1028,26 +1028,28 @@ static int pk_parse_key_pkcs8_unencrypted_der(
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
 
     if( ( ret = mbedtls_asn1_get_int( &p, end, &version ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( version != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_VERSION + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_VERSION, ret ) );
 
     if( ( ret = pk_get_pk_alg( &p, end, &pk_alg, &params ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+    {
+        return( ret );
+    }
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( len < 1 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     if( ( pk_info = mbedtls_pk_info_from_type( pk_alg ) ) == NULL )
         return( MBEDTLS_ERR_PK_UNKNOWN_PK_ALG );
@@ -1130,16 +1132,16 @@ static int pk_parse_key_pkcs8_encrypted_der(
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
 
     if( ( ret = mbedtls_asn1_get_alg( &p, end, &pbe_alg_oid, &pbe_params ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len, MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret ) );
 
     buf = p;
 
@@ -1518,7 +1520,8 @@ int mbedtls_pk_parse_public_key( mbedtls_pk_context *ctx,
         return( ret );
     }
     mbedtls_pk_free( ctx );
-    if( ret != ( MBEDTLS_ERR_PK_INVALID_PUBKEY + MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) )
+    if( ret != ( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_PK_INVALID_PUBKEY,
+                                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) ) )
     {
         return( ret );
     }
diff --git a/library/rsa.c b/library/rsa.c
index 32eb74e2a9..209273e090 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -102,7 +102,7 @@ int mbedtls_rsa_import( mbedtls_rsa_context *ctx,
         ( D != NULL && ( ret = mbedtls_mpi_copy( &ctx->D, D ) ) != 0 ) ||
         ( E != NULL && ( ret = mbedtls_mpi_copy( &ctx->E, E ) ) != 0 ) )
     {
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
     }
 
     if( N != NULL )
@@ -142,7 +142,7 @@ int mbedtls_rsa_import_raw( mbedtls_rsa_context *ctx,
 cleanup:
 
     if( ret != 0 )
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
 
     return( 0 );
 }
@@ -293,7 +293,7 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
         if( ( ret = mbedtls_mpi_mul_mpi( &ctx->N, &ctx->P,
                                          &ctx->Q ) ) != 0 )
         {
-            return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
         }
 
         ctx->len = mbedtls_mpi_size( &ctx->N );
@@ -308,7 +308,7 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
         ret = mbedtls_rsa_deduce_primes( &ctx->N, &ctx->E, &ctx->D,
                                          &ctx->P, &ctx->Q );
         if( ret != 0 )
-            return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
 
     }
     else if( d_missing )
@@ -318,7 +318,7 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
                                                          &ctx->E,
                                                          &ctx->D ) ) != 0 )
         {
-            return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
         }
     }
 
@@ -333,7 +333,7 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
         ret = mbedtls_rsa_deduce_crt( &ctx->P,  &ctx->Q,  &ctx->D,
                                       &ctx->DP, &ctx->DQ, &ctx->QP );
         if( ret != 0 )
-            return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
     }
 #endif /* MBEDTLS_RSA_NO_CRT */
 
@@ -461,13 +461,13 @@ int mbedtls_rsa_export_crt( const mbedtls_rsa_context *ctx,
         ( DQ != NULL && ( ret = mbedtls_mpi_copy( DQ, &ctx->DQ ) ) != 0 ) ||
         ( QP != NULL && ( ret = mbedtls_mpi_copy( QP, &ctx->QP ) ) != 0 ) )
     {
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
     }
 #else
     if( ( ret = mbedtls_rsa_deduce_crt( &ctx->P, &ctx->Q, &ctx->D,
                                         DP, DQ, QP ) ) != 0 )
     {
-        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_BAD_INPUT_DATA, ret ) );
     }
 #endif
 
@@ -635,8 +635,9 @@ cleanup:
     if( ret != 0 )
     {
         mbedtls_rsa_free( ctx );
+
         if( ( -ret & ~0x7f ) == 0 )
-            ret = MBEDTLS_ERR_RSA_KEY_GEN_FAILED + ret;
+            ret = MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_KEY_GEN_FAILED, ret );
         return( ret );
     }
 
@@ -769,7 +770,7 @@ cleanup:
     mbedtls_mpi_free( &T );
 
     if( ret != 0 )
-        return( MBEDTLS_ERR_RSA_PUBLIC_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_PUBLIC_FAILED, ret ) );
 
     return( 0 );
 }
@@ -1085,7 +1086,7 @@ cleanup:
     mbedtls_mpi_free( &I );
 
     if( ret != 0 && ret >= -0x007f )
-        return( MBEDTLS_ERR_RSA_PRIVATE_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_PRIVATE_FAILED, ret ) );
 
     return( ret );
 }
@@ -1198,7 +1199,7 @@ int mbedtls_rsa_rsaes_oaep_encrypt( mbedtls_rsa_context *ctx,
 
     /* Generate a random octet string seed */
     if( ( ret = f_rng( p_rng, p, hlen ) ) != 0 )
-        return( MBEDTLS_ERR_RSA_RNG_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_RNG_FAILED, ret ) );
 
     p += hlen;
 
@@ -1287,7 +1288,7 @@ int mbedtls_rsa_rsaes_pkcs1_v15_encrypt( mbedtls_rsa_context *ctx,
 
             /* Check if RNG failed to generate data */
             if( rng_dl == 0 || ret != 0 )
-                return( MBEDTLS_ERR_RSA_RNG_FAILED + ret );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_RNG_FAILED, ret ) );
 
             p++;
         }
@@ -1881,7 +1882,7 @@ static int rsa_rsassa_pss_sign( mbedtls_rsa_context *ctx,
     /* Generate salt of length slen in place in the encoded message */
     salt = p;
     if( ( ret = f_rng( p_rng, salt, slen ) ) != 0 )
-        return( MBEDTLS_ERR_RSA_RNG_FAILED + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_RSA_RNG_FAILED, ret ) );
 
     p += slen;
 
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index fa89a07d34..1cfab87192 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -174,7 +174,7 @@ int mbedtls_ssl_cookie_write( void *p_ctx,
 
 #if defined(MBEDTLS_THREADING_C)
     if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR, ret ) );
 #endif
 
     ret = ssl_cookie_hmac( &ctx->hmac_ctx, *p - 4,
@@ -182,8 +182,8 @@ int mbedtls_ssl_cookie_write( void *p_ctx,
 
 #if defined(MBEDTLS_THREADING_C)
     if( mbedtls_mutex_unlock( &ctx->mutex ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR +
-                MBEDTLS_ERR_THREADING_MUTEX_ERROR );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR,
+                MBEDTLS_ERR_THREADING_MUTEX_ERROR ) );
 #endif
 
     return( ret );
@@ -210,7 +210,7 @@ int mbedtls_ssl_cookie_check( void *p_ctx,
 
 #if defined(MBEDTLS_THREADING_C)
     if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR, ret ) );
 #endif
 
     if( ssl_cookie_hmac( &ctx->hmac_ctx, cookie,
@@ -220,8 +220,8 @@ int mbedtls_ssl_cookie_check( void *p_ctx,
 
 #if defined(MBEDTLS_THREADING_C)
     if( mbedtls_mutex_unlock( &ctx->mutex ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR +
-                MBEDTLS_ERR_THREADING_MUTEX_ERROR );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR,
+                MBEDTLS_ERR_THREADING_MUTEX_ERROR ) );
 #endif
 
     if( ret != 0 )
diff --git a/library/x509.c b/library/x509.c
index f0a9101e5f..b649d3cdce 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -81,18 +81,18 @@ int mbedtls_x509_get_serial( unsigned char **p, const unsigned char *end,
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     if( ( end - *p ) < 1 )
-        return( MBEDTLS_ERR_X509_INVALID_SERIAL +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_SERIAL,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     if( **p != ( MBEDTLS_ASN1_CONTEXT_SPECIFIC | MBEDTLS_ASN1_PRIMITIVE | 2 ) &&
         **p !=   MBEDTLS_ASN1_INTEGER )
-        return( MBEDTLS_ERR_X509_INVALID_SERIAL +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_SERIAL,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     serial->tag = *(*p)++;
 
     if( ( ret = mbedtls_asn1_get_len( p, end, &serial->len ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_SERIAL + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_SERIAL, ret ) );
 
     serial->p = *p;
     *p += serial->len;
@@ -112,7 +112,7 @@ int mbedtls_x509_get_alg_null( unsigned char **p, const unsigned char *end,
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     if( ( ret = mbedtls_asn1_get_alg_null( p, end, alg ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     return( 0 );
 }
@@ -126,7 +126,7 @@ int mbedtls_x509_get_alg( unsigned char **p, const unsigned char *end,
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     if( ( ret = mbedtls_asn1_get_alg( p, end, alg, params ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     return( 0 );
 }
@@ -151,39 +151,39 @@ static int x509_get_hash_alg( const mbedtls_x509_buf *alg, mbedtls_md_type_t *md
 
     /* Make sure we got a SEQUENCE and setup bounds */
     if( alg->tag != ( MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) )
-        return( MBEDTLS_ERR_X509_INVALID_ALG +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     p = alg->p;
     end = p + alg->len;
 
     if( p >= end )
-        return( MBEDTLS_ERR_X509_INVALID_ALG +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     /* Parse md_oid */
     md_oid.tag = *p;
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &md_oid.len, MBEDTLS_ASN1_OID ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     md_oid.p = p;
     p += md_oid.len;
 
     /* Get md_alg from md_oid */
     if( ( ret = mbedtls_oid_get_md_alg( &md_oid, md_alg ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     /* Make sure params is absent of NULL */
     if( p == end )
         return( 0 );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len, MBEDTLS_ASN1_NULL ) ) != 0 || len != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     if( p != end )
-        return( MBEDTLS_ERR_X509_INVALID_ALG +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -217,8 +217,8 @@ int mbedtls_x509_get_rsassa_pss_params( const mbedtls_x509_buf *params,
 
     /* Make sure params is a SEQUENCE and setup bounds */
     if( params->tag != ( MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) )
-        return( MBEDTLS_ERR_X509_INVALID_ALG +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     p = (unsigned char *) params->p;
     end = p + params->len;
@@ -239,14 +239,14 @@ int mbedtls_x509_get_rsassa_pss_params( const mbedtls_x509_buf *params,
             return( ret );
 
         if( ( ret = mbedtls_oid_get_md_alg( &alg_id, md_alg ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
         if( p != end2 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
     else if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     if( p == end )
         return( 0 );
@@ -265,19 +265,19 @@ int mbedtls_x509_get_rsassa_pss_params( const mbedtls_x509_buf *params,
 
         /* Only MFG1 is recognised for now */
         if( MBEDTLS_OID_CMP( MBEDTLS_OID_MGF1, &alg_id ) != 0 )
-            return( MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE +
-                    MBEDTLS_ERR_OID_NOT_FOUND );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE,
+                    MBEDTLS_ERR_OID_NOT_FOUND ) );
 
         /* Parse HashAlgorithm */
         if( ( ret = x509_get_hash_alg( &alg_params, mgf_md ) ) != 0 )
             return( ret );
 
         if( p != end2 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
     else if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     if( p == end )
         return( 0 );
@@ -291,14 +291,14 @@ int mbedtls_x509_get_rsassa_pss_params( const mbedtls_x509_buf *params,
         end2 = p + len;
 
         if( ( ret = mbedtls_asn1_get_int( &p, end2, salt_len ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
         if( p != end2 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
     else if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     if( p == end )
         return( 0 );
@@ -314,21 +314,21 @@ int mbedtls_x509_get_rsassa_pss_params( const mbedtls_x509_buf *params,
         end2 = p + len;
 
         if( ( ret = mbedtls_asn1_get_int( &p, end2, &trailer_field ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
         if( p != end2 )
-            return( MBEDTLS_ERR_X509_INVALID_ALG +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         if( trailer_field != 1 )
             return( MBEDTLS_ERR_X509_INVALID_ALG );
     }
     else if( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
-        return( MBEDTLS_ERR_X509_INVALID_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG, ret ) );
 
     if( p != end )
-        return( MBEDTLS_ERR_X509_INVALID_ALG +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_ALG,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -354,47 +354,47 @@ static int x509_get_attr_type_value( unsigned char **p,
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_NAME + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME, ret ) );
 
     end = *p + len;
 
     if( ( end - *p ) < 1 )
-        return( MBEDTLS_ERR_X509_INVALID_NAME +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     oid = &cur->oid;
     oid->tag = **p;
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &oid->len, MBEDTLS_ASN1_OID ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_NAME + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME, ret ) );
 
     oid->p = *p;
     *p += oid->len;
 
     if( ( end - *p ) < 1 )
-        return( MBEDTLS_ERR_X509_INVALID_NAME +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     if( **p != MBEDTLS_ASN1_BMP_STRING && **p != MBEDTLS_ASN1_UTF8_STRING      &&
         **p != MBEDTLS_ASN1_T61_STRING && **p != MBEDTLS_ASN1_PRINTABLE_STRING &&
         **p != MBEDTLS_ASN1_IA5_STRING && **p != MBEDTLS_ASN1_UNIVERSAL_STRING &&
         **p != MBEDTLS_ASN1_BIT_STRING )
-        return( MBEDTLS_ERR_X509_INVALID_NAME +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     val = &cur->val;
     val->tag = *(*p)++;
 
     if( ( ret = mbedtls_asn1_get_len( p, end, &val->len ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_NAME + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME, ret ) );
 
     val->p = *p;
     *p += val->len;
 
     if( *p != end )
     {
-        return( MBEDTLS_ERR_X509_INVALID_NAME +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     cur->next = NULL;
@@ -440,7 +440,7 @@ int mbedtls_x509_get_name( unsigned char **p, const unsigned char *end,
          */
         if( ( ret = mbedtls_asn1_get_tag( p, end, &set_len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SET ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_NAME + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_NAME, ret ) );
 
         end_set  = *p + set_len;
 
@@ -604,8 +604,8 @@ int mbedtls_x509_get_time( unsigned char **p, const unsigned char *end,
     unsigned char tag;
 
     if( ( end - *p ) < 1 )
-        return( MBEDTLS_ERR_X509_INVALID_DATE +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     tag = **p;
 
@@ -614,14 +614,14 @@ int mbedtls_x509_get_time( unsigned char **p, const unsigned char *end,
     else if( tag == MBEDTLS_ASN1_GENERALIZED_TIME )
         year_len = 4;
     else
-        return( MBEDTLS_ERR_X509_INVALID_DATE +
-                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 
     (*p)++;
     ret = mbedtls_asn1_get_len( p, end, &len );
 
     if( ret != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_DATE + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE, ret ) );
 
     return x509_parse_time( p, len, year_len, tm );
 }
@@ -633,13 +633,13 @@ int mbedtls_x509_get_sig( unsigned char **p, const unsigned char *end, mbedtls_x
     int tag_type;
 
     if( ( end - *p ) < 1 )
-        return( MBEDTLS_ERR_X509_INVALID_SIGNATURE +
-                MBEDTLS_ERR_ASN1_OUT_OF_DATA );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_SIGNATURE,
+                MBEDTLS_ERR_ASN1_OUT_OF_DATA ) );
 
     tag_type = **p;
 
     if( ( ret = mbedtls_asn1_get_bitstring_null( p, end, &len ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_SIGNATURE + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_SIGNATURE, ret ) );
 
     sig->tag = tag_type;
     sig->len = len;
@@ -663,7 +663,7 @@ int mbedtls_x509_get_sig_alg( const mbedtls_x509_buf *sig_oid, const mbedtls_x50
         return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
 
     if( ( ret = mbedtls_oid_get_sig_alg( sig_oid, md_alg, pk_alg ) ) != 0 )
-        return( MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG, ret ) );
 
 #if defined(MBEDTLS_X509_RSASSA_PSS_SUPPORT)
     if( *pk_alg == MBEDTLS_PK_RSASSA_PSS )
@@ -714,7 +714,7 @@ int mbedtls_x509_get_ext( unsigned char **p, const unsigned char *end,
     ret = mbedtls_asn1_get_tag( p, end, &ext->len,
               MBEDTLS_ASN1_CONTEXT_SPECIFIC | MBEDTLS_ASN1_CONSTRUCTED | tag );
     if( ret != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     ext->tag = MBEDTLS_ASN1_CONTEXT_SPECIFIC | MBEDTLS_ASN1_CONSTRUCTED | tag;
     ext->p   = *p;
@@ -725,11 +725,11 @@ int mbedtls_x509_get_ext( unsigned char **p, const unsigned char *end,
      */
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( end != *p + len )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
diff --git a/library/x509_crl.c b/library/x509_crl.c
index edeb39b02d..ac4fc75de3 100644
--- a/library/x509_crl.c
+++ b/library/x509_crl.c
@@ -79,7 +79,7 @@ static int x509_crl_get_version( unsigned char **p,
             return( 0 );
         }
 
-        return( MBEDTLS_ERR_X509_INVALID_VERSION + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_VERSION, ret ) );
     }
 
     return( 0 );
@@ -125,7 +125,7 @@ static int x509_get_crl_ext( unsigned char **p,
         /* Get enclosing sequence tag */
         if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         end_ext_data = *p + len;
 
@@ -133,7 +133,7 @@ static int x509_get_crl_ext( unsigned char **p,
         if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &len,
                                           MBEDTLS_ASN1_OID ) ) != 0 )
         {
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
         }
         *p += len;
 
@@ -142,29 +142,29 @@ static int x509_get_crl_ext( unsigned char **p,
                                            &is_critical ) ) != 0 &&
             ( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) )
         {
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
         }
 
         /* Data should be octet string type */
         if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &len,
                 MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         /* Ignore data so far and just check its length */
         *p += len;
         if( *p != end_ext_data )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         /* Abort on (unsupported) critical extensions */
         if( is_critical )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
     }
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -198,27 +198,27 @@ static int x509_get_crl_entry_ext( unsigned char **p,
             ext->p = NULL;
             return( 0 );
         }
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
     }
 
     end = *p + ext->len;
 
     if( end != *p + ext->len )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     while( *p < end )
     {
         if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         *p += len;
     }
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -364,8 +364,8 @@ int mbedtls_x509_crl_parse_der( mbedtls_x509_crl *chain,
     if( len != (size_t) ( end - p ) )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     /*
@@ -377,7 +377,7 @@ int mbedtls_x509_crl_parse_der( mbedtls_x509_crl *chain,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
@@ -421,7 +421,7 @@ int mbedtls_x509_crl_parse_der( mbedtls_x509_crl *chain,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     if( ( ret = mbedtls_x509_get_name( &p, p + len, &crl->issuer ) ) != 0 )
@@ -444,10 +444,10 @@ int mbedtls_x509_crl_parse_der( mbedtls_x509_crl *chain,
 
     if( ( ret = mbedtls_x509_get_time( &p, end, &crl->next_update ) ) != 0 )
     {
-        if( ret != ( MBEDTLS_ERR_X509_INVALID_DATE +
-                        MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) &&
-            ret != ( MBEDTLS_ERR_X509_INVALID_DATE +
-                        MBEDTLS_ERR_ASN1_OUT_OF_DATA ) )
+        if( ret != ( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                        MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) ) &&
+            ret != ( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                        MBEDTLS_ERR_ASN1_OUT_OF_DATA ) ) )
         {
             mbedtls_x509_crl_free( crl );
             return( ret );
@@ -486,8 +486,8 @@ int mbedtls_x509_crl_parse_der( mbedtls_x509_crl *chain,
     if( p != end )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     end = crl->raw.p + crl->raw.len;
@@ -521,8 +521,8 @@ int mbedtls_x509_crl_parse_der( mbedtls_x509_crl *chain,
     if( p != end )
     {
         mbedtls_x509_crl_free( crl );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     return( 0 );
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 783f3ba5c3..aa5506a568 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -393,17 +393,17 @@ static int x509_get_version( unsigned char **p,
             return( 0 );
         }
 
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     end = *p + len;
 
     if( ( ret = mbedtls_asn1_get_int( p, end, ver ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_VERSION + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_VERSION, ret ) );
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_VERSION +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_VERSION,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -423,7 +423,7 @@ static int x509_get_dates( unsigned char **p,
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_DATE + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE, ret ) );
 
     end = *p + len;
 
@@ -434,8 +434,8 @@ static int x509_get_dates( unsigned char **p,
         return( ret );
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_DATE +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_DATE,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -460,7 +460,7 @@ static int x509_get_uid( unsigned char **p,
         if( ret == MBEDTLS_ERR_ASN1_UNEXPECTED_TAG )
             return( 0 );
 
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     uid->p = *p;
@@ -487,7 +487,7 @@ static int x509_get_basic_constraints( unsigned char **p,
 
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( *p == end )
         return( 0 );
@@ -498,7 +498,7 @@ static int x509_get_basic_constraints( unsigned char **p,
             ret = mbedtls_asn1_get_int( p, end, ca_istrue );
 
         if( ret != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         if( *ca_istrue != 0 )
             *ca_istrue = 1;
@@ -508,17 +508,17 @@ static int x509_get_basic_constraints( unsigned char **p,
         return( 0 );
 
     if( ( ret = mbedtls_asn1_get_int( p, end, max_pathlen ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     /* Do not accept max_pathlen equal to INT_MAX to avoid a signed integer
      * overflow, which is an undefined behavior. */
     if( *max_pathlen == INT_MAX )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_INVALID_LENGTH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_INVALID_LENGTH ) );
 
     (*max_pathlen)++;
 
@@ -533,11 +533,11 @@ static int x509_get_ns_cert_type( unsigned char **p,
     mbedtls_x509_bitstring bs = { 0, 0, NULL };
 
     if( ( ret = mbedtls_asn1_get_bitstring( p, end, &bs ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( bs.len != 1 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_INVALID_LENGTH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_INVALID_LENGTH ) );
 
     /* Get actual bitstring */
     *ns_cert_type = *bs.p;
@@ -553,11 +553,11 @@ static int x509_get_key_usage( unsigned char **p,
     mbedtls_x509_bitstring bs = { 0, 0, NULL };
 
     if( ( ret = mbedtls_asn1_get_bitstring( p, end, &bs ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( bs.len < 1 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_INVALID_LENGTH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_INVALID_LENGTH ) );
 
     /* Get actual bitstring */
     *key_usage = 0;
@@ -581,12 +581,12 @@ static int x509_get_ext_key_usage( unsigned char **p,
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     if( ( ret = mbedtls_asn1_get_sequence_of( p, end, ext_key_usage, MBEDTLS_ASN1_OID ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     /* Sequence length must be >= 1 */
     if( ext_key_usage->buf.p == NULL )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_INVALID_LENGTH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_INVALID_LENGTH ) );
 
     return( 0 );
 }
@@ -631,11 +631,11 @@ static int x509_get_subject_alt_name( unsigned char **p,
     /* Get main sequence tag */
     if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( *p + len != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     while( *p < end )
     {
@@ -645,13 +645,13 @@ static int x509_get_subject_alt_name( unsigned char **p,
         tag = **p;
         (*p)++;
         if( ( ret = mbedtls_asn1_get_len( p, end, &tag_len ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         if( ( tag & MBEDTLS_ASN1_TAG_CLASS_MASK ) !=
                 MBEDTLS_ASN1_CONTEXT_SPECIFIC )
         {
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
         }
 
         /*
@@ -687,8 +687,8 @@ static int x509_get_subject_alt_name( unsigned char **p,
             cur->next = mbedtls_calloc( 1, sizeof( mbedtls_asn1_sequence ) );
 
             if( cur->next == NULL )
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                        MBEDTLS_ERR_ASN1_ALLOC_FAILED );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                        MBEDTLS_ERR_ASN1_ALLOC_FAILED ) );
 
             cur = cur->next;
         }
@@ -704,8 +704,8 @@ static int x509_get_subject_alt_name( unsigned char **p,
     cur->next = NULL;
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -772,18 +772,18 @@ static int x509_get_certificate_policies( unsigned char **p,
     ret = mbedtls_asn1_get_tag( p, end, &len,
                              MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE );
     if( ret != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( *p + len != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     /*
      * Cannot be an empty sequence.
      */
     if( len == 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     while( *p < end )
     {
@@ -795,13 +795,13 @@ static int x509_get_certificate_policies( unsigned char **p,
          */
         if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         policy_end = *p + len;
 
         if( ( ret = mbedtls_asn1_get_tag( p, policy_end, &len,
                                           MBEDTLS_ASN1_OID ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         policy_oid.tag = MBEDTLS_ASN1_OID;
         policy_oid.len = len;
@@ -829,8 +829,8 @@ static int x509_get_certificate_policies( unsigned char **p,
             cur->next = mbedtls_calloc( 1, sizeof( mbedtls_asn1_sequence ) );
 
             if( cur->next == NULL )
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                        MBEDTLS_ERR_ASN1_ALLOC_FAILED );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                        MBEDTLS_ERR_ASN1_ALLOC_FAILED ) );
 
             cur = cur->next;
         }
@@ -850,7 +850,7 @@ static int x509_get_certificate_policies( unsigned char **p,
         {
             if( ( ret = mbedtls_asn1_get_tag( p, policy_end, &len,
                      MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
             /*
              * Skip the optional policy qualifiers.
              */
@@ -858,16 +858,16 @@ static int x509_get_certificate_policies( unsigned char **p,
         }
 
         if( *p != policy_end )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     /* Set final sequence entry's next pointer to NULL */
     cur->next = NULL;
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( parse_ret );
 }
@@ -907,14 +907,14 @@ static int x509_get_crt_ext( unsigned char **p,
 
         if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                 MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         end_ext_data = *p + len;
 
         /* Get extension ID */
         if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &extn_oid.len,
                                           MBEDTLS_ASN1_OID ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         extn_oid.tag = MBEDTLS_ASN1_OID;
         extn_oid.p = *p;
@@ -923,19 +923,19 @@ static int x509_get_crt_ext( unsigned char **p,
         /* Get optional critical */
         if( ( ret = mbedtls_asn1_get_bool( p, end_ext_data, &is_critical ) ) != 0 &&
             ( ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         /* Data should be octet string type */
         if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &len,
                 MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         start_ext_octet = *p;
         end_ext_octet = *p + len;
 
         if( end_ext_octet != end_ext_data )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         /*
          * Detect supported extensions
@@ -961,8 +961,8 @@ static int x509_get_crt_ext( unsigned char **p,
             if( is_critical )
             {
                 /* Data is marked as critical: fail */
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                        MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                        MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
             }
 #endif
             continue;
@@ -1055,8 +1055,8 @@ static int x509_get_crt_ext( unsigned char **p,
     }
 
     if( *p != end )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
     return( 0 );
 }
@@ -1134,7 +1134,7 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crt_free( crt );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
@@ -1181,7 +1181,7 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crt_free( crt );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     if( ( ret = mbedtls_x509_get_name( &p, p + len, &crt->issuer ) ) != 0 )
@@ -1214,7 +1214,7 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_crt_free( crt );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     if( len && ( ret = mbedtls_x509_get_name( &p, p + len, &crt->subject ) ) != 0 )
@@ -1279,8 +1279,8 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
     if( p != end )
     {
         mbedtls_x509_crt_free( crt );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     end = crt_end;
@@ -1318,8 +1318,8 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
     if( p != end )
     {
         mbedtls_x509_crt_free( crt );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     return( 0 );
@@ -1702,7 +1702,7 @@ static int x509_get_other_name( const mbedtls_x509_buf *subject_alt_name,
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
                                       MBEDTLS_ASN1_OID ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     cur_oid.tag = MBEDTLS_ASN1_OID;
     cur_oid.p = p;
@@ -1719,20 +1719,20 @@ static int x509_get_other_name( const mbedtls_x509_buf *subject_alt_name,
     if( p + len >= end )
     {
         mbedtls_platform_zeroize( other_name, sizeof( *other_name ) );
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
     p += len;
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CONTEXT_SPECIFIC ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
                      MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-       return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+       return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len, MBEDTLS_ASN1_OID ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     other_name->value.hardware_module_name.oid.tag = MBEDTLS_ASN1_OID;
     other_name->value.hardware_module_name.oid.p = p;
@@ -1741,13 +1741,13 @@ static int x509_get_other_name( const mbedtls_x509_buf *subject_alt_name,
     if( p + len >= end )
     {
         mbedtls_platform_zeroize( other_name, sizeof( *other_name ) );
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
     p += len;
     if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
                                       MBEDTLS_ASN1_OCTET_STRING ) ) != 0 )
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
     other_name->value.hardware_module_name.val.tag = MBEDTLS_ASN1_OCTET_STRING;
     other_name->value.hardware_module_name.val.p = p;
@@ -1757,8 +1757,8 @@ static int x509_get_other_name( const mbedtls_x509_buf *subject_alt_name,
     {
         mbedtls_platform_zeroize( other_name,
                                   sizeof( *other_name ) );
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
     return( 0 );
 }
diff --git a/library/x509_csr.c b/library/x509_csr.c
index 5463f8a9e0..e259410d07 100644
--- a/library/x509_csr.c
+++ b/library/x509_csr.c
@@ -73,7 +73,7 @@ static int x509_csr_get_version( unsigned char **p,
             return( 0 );
         }
 
-        return( MBEDTLS_ERR_X509_INVALID_VERSION + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_VERSION, ret ) );
     }
 
     return( 0 );
@@ -131,8 +131,8 @@ int mbedtls_x509_csr_parse_der( mbedtls_x509_csr *csr,
     if( len != (size_t) ( end - p ) )
     {
         mbedtls_x509_csr_free( csr );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     /*
@@ -144,7 +144,7 @@ int mbedtls_x509_csr_parse_der( mbedtls_x509_csr *csr,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_csr_free( csr );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     end = p + len;
@@ -176,7 +176,7 @@ int mbedtls_x509_csr_parse_der( mbedtls_x509_csr *csr,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
     {
         mbedtls_x509_csr_free( csr );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     if( ( ret = mbedtls_x509_get_name( &p, p + len, &csr->subject ) ) != 0 )
@@ -210,7 +210,7 @@ int mbedtls_x509_csr_parse_der( mbedtls_x509_csr *csr,
             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_CONTEXT_SPECIFIC ) ) != 0 )
     {
         mbedtls_x509_csr_free( csr );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT, ret ) );
     }
 
     p += len;
@@ -244,8 +244,8 @@ int mbedtls_x509_csr_parse_der( mbedtls_x509_csr *csr,
     if( p != end )
     {
         mbedtls_x509_csr_free( csr );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_FORMAT,
+                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
     }
 
     return( 0 );
diff --git a/scripts/data_files/error.fmt b/scripts/data_files/error.fmt
index 9e479bbfd9..3be94bd2c4 100644
--- a/scripts/data_files/error.fmt
+++ b/scripts/data_files/error.fmt
@@ -162,4 +162,8 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
 
 #endif /* MBEDTLS_ERROR_C */
 
+#if defined(MBEDTLS_TEST_HOOKS)
+void (*mbedtls_test_hook_error_add)( int, int, const char *, int );
+#endif
+
 #endif /* MBEDTLS_ERROR_C || MBEDTLS_ERROR_STRERROR_DUMMY */
diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index c3a844b600..9bfe08547c 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -278,4 +278,20 @@ void mbedtls_test_mutex_usage_init( void );
 void mbedtls_test_mutex_usage_check( void );
 #endif /* MBEDTLS_TEST_MUTEX_USAGE */
 
+#if defined(MBEDTLS_TEST_HOOKS)
+/**
+ * \brief   Check that only a pure high-level error code is being combined with
+ *          a pure low-level error code as otherwise the resultant error code
+ *          would be corrupted.
+ *
+ * \note    Both high-level and low-level error codes cannot be greater than
+ *          zero however can be zero. If one error code is zero then the
+ *          other error code is returned even if both codes are zero.
+ *
+ * \note    If the check fails, fail the test currently being run.
+ */
+void mbedtls_test_err_add_check( int high, int low,
+                                 const char *file, int line);
+#endif
+
 #endif /* TEST_HELPERS_H */
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index e323275e5f..b7c9867b05 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -282,3 +282,59 @@ void mbedtls_param_failed( const char *failure_condition,
     }
 }
 #endif /* MBEDTLS_CHECK_PARAMS */
+
+#if defined(MBEDTLS_TEST_HOOKS)
+void mbedtls_test_err_add_check( int high, int low,
+                                 const char *file, int line )
+{
+    /* Error codes are always negative (a value of zero is a success) however
+     * their positive opposites can be easier to understand. The following
+     * examples given in comments have been made positive for ease of
+     * understanding. The structure of an error code is such:
+     *
+     *                                                shhhhhhhhlllllll
+     *
+     * s = sign bit.
+     * h = high level error code (includes high level module ID (bits 12..14)
+     *     and module-dependent error code (bits 7..11)).
+     * l = low level error code.
+     */
+    if ( high > -0x1000 && high != 0 )
+    /* high < 0001000000000000
+     * No high level module ID bits are set.
+     */
+    {
+        mbedtls_test_fail( "'high' is not a high-level error code",
+                            line, file );
+    }
+    else if ( high < -0x7F80 )
+    /* high > 0111111110000000
+     * Error code is greater than the largest allowed high level module ID.
+     */
+    {
+        mbedtls_test_fail( "'high' error code is greater than 15 bits",
+                            line, file );
+    }
+    else if ( ( high & 0x7F ) != 0 )
+    /* high & 0000000001111111
+     * Error code contains low level error code bits.
+     */
+    {
+        mbedtls_test_fail( "'high' contains a low-level error code",
+                            line, file );
+    }
+    else if ( low < -0x007F )
+    /* low >  0000000001111111
+     * Error code contains high or module level error code bits.
+     */
+    {
+        mbedtls_test_fail( "'low' error code is greater than 7 bits",
+                            line, file );
+    }
+    else if ( low > 0 )
+    {
+        mbedtls_test_fail( "'low' error code is greater than zero",
+                            line, file );
+    }
+}
+#endif /* MBEDTLS_TEST_HOOKS */
diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index 36a7d231e3..ac00f45e57 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -33,6 +33,10 @@
 #include "psa/crypto.h"
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
+#if defined(MBEDTLS_TEST_HOOKS)
+#include "mbedtls/error.h"
+#endif
+
 /* Test code may use deprecated identifiers only if the preprocessor symbol
  * MBEDTLS_TEST_DEPRECATED is defined. When building tests, set
  * MBEDTLS_TEST_DEPRECATED explicitly if MBEDTLS_DEPRECATED_WARNING is
@@ -279,6 +283,10 @@ $platform_code
  */
 int main( int argc, const char *argv[] )
 {
+#if defined(MBEDTLS_TEST_HOOKS) && defined (MBEDTLS_ERROR_C)
+    mbedtls_test_hook_error_add = &mbedtls_test_err_add_check;
+#endif
+
     int ret = mbedtls_test_platform_setup();
     if( ret != 0 )
     {
diff --git a/tests/suites/test_suite_x509parse.function b/tests/suites/test_suite_x509parse.function
index 46d97240d4..f31948d8cc 100644
--- a/tests/suites/test_suite_x509parse.function
+++ b/tests/suites/test_suite_x509parse.function
@@ -7,6 +7,7 @@
 #include "mbedtls/pem.h"
 #include "mbedtls/oid.h"
 #include "mbedtls/base64.h"
+#include "mbedtls/error.h"
 #include "string.h"
 
 #if MBEDTLS_X509_MAX_INTERMEDIATE_CA > 19
@@ -320,18 +321,18 @@ int parse_crt_ext_cb( void *p_ctx, mbedtls_x509_crt const *crt, mbedtls_x509_buf
         ret = mbedtls_asn1_get_tag( p, end, &len,
                                  MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE );
         if( ret != 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
         if( *p + len != end )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         /*
          * Cannot be an empty sequence.
          */
         if( len == 0 )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         while( *p < end )
         {
@@ -342,13 +343,13 @@ int parse_crt_ext_cb( void *p_ctx, mbedtls_x509_crt const *crt, mbedtls_x509_buf
              */
             if( ( ret = mbedtls_asn1_get_tag( p, end, &len,
                     MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
             policy_end = *p + len;
 
             if( ( ret = mbedtls_asn1_get_tag( p, policy_end, &len,
                                               MBEDTLS_ASN1_OID ) ) != 0 )
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
 
             /*
              * Recognize exclusively the policy with OID 1
@@ -366,7 +367,7 @@ int parse_crt_ext_cb( void *p_ctx, mbedtls_x509_crt const *crt, mbedtls_x509_buf
             {
                 if( ( ret = mbedtls_asn1_get_tag( p, policy_end, &len,
                          MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) ) != 0 )
-                    return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
+                    return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret ) );
                 /*
                  * Skip the optional policy qualifiers.
                  */
@@ -374,13 +375,13 @@ int parse_crt_ext_cb( void *p_ctx, mbedtls_x509_crt const *crt, mbedtls_x509_buf
             }
 
             if( *p != policy_end )
-                return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                        MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+                return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                        MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
         }
 
         if( *p != end )
-            return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
+            return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                    MBEDTLS_ERR_ASN1_LENGTH_MISMATCH ) );
 
         return( parse_ret );
     }
@@ -388,7 +389,8 @@ int parse_crt_ext_cb( void *p_ctx, mbedtls_x509_crt const *crt, mbedtls_x509_buf
              memcmp( new_oid->p, oid->p, oid->len ) == 0 )
         return( 0 );
     else
-        return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                                   MBEDTLS_ERR_ASN1_UNEXPECTED_TAG ) );
 }
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 /* END_HEADER */