diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index 154a6ba201..dc30206ba1 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -83,7 +83,7 @@ int asn1_write_raw_buffer( unsigned char **p, unsigned char *start,
  *
  * \return          the length written or a negative error code
  */
-int asn1_write_mpi( unsigned char **p, unsigned char *start, mpi *X );
+int asn1_write_mpi( unsigned char **p, unsigned char *start, const mpi *X );
 #endif /* POLARSSL_BIGNUM_C */
 
 /**
diff --git a/library/asn1write.c b/library/asn1write.c
index 78038675d9..a5a35f60a9 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -99,7 +99,7 @@ int asn1_write_raw_buffer( unsigned char **p, unsigned char *start,
 }
 
 #if defined(POLARSSL_BIGNUM_C)
-int asn1_write_mpi( unsigned char **p, unsigned char *start, mpi *X )
+int asn1_write_mpi( unsigned char **p, unsigned char *start, const mpi *X )
 {
     int ret;
     size_t len = 0;