diff --git a/library/bignum_core.c b/library/bignum_core.c
index 463f145148..2e2df37bb4 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -746,7 +746,7 @@ static void exp_mod_precompute_window(const mbedtls_mpi_uint *A,
     }
 }
 
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
 // Set to a default that is neither MBEDTLS_MPI_IS_PUBLIC nor MBEDTLS_MPI_IS_SECRET
 int mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_PUBLIC + MBEDTLS_MPI_IS_SECRET + 1;
 #endif
@@ -770,7 +770,7 @@ static inline void exp_mod_calc_first_bit_optionally_safe(const mbedtls_mpi_uint
             *E_limb_index = E_bits / biL;
             *E_bit_index = E_bits % biL;
         }
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
         mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_PUBLIC;
 #endif
     } else {
@@ -780,7 +780,7 @@ static inline void exp_mod_calc_first_bit_optionally_safe(const mbedtls_mpi_uint
          */
         *E_limb_index = E_limbs;
         *E_bit_index = 0;
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
         // Only mark the codepath safe if there wasn't an unsafe codepath before
         if (mbedtls_mpi_optionally_safe_codepath != MBEDTLS_MPI_IS_PUBLIC) {
             mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_SECRET;
@@ -802,7 +802,7 @@ static inline void exp_mod_table_lookup_optionally_safe(mbedtls_mpi_uint *Wselec
 {
     if (window_public == MBEDTLS_MPI_IS_PUBLIC) {
         memcpy(Wselect, Wtable + window * AN_limbs, AN_limbs * ciL);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
         mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_PUBLIC;
 #endif
     } else {
@@ -810,7 +810,7 @@ static inline void exp_mod_table_lookup_optionally_safe(mbedtls_mpi_uint *Wselec
          * memory access patterns. */
         mbedtls_mpi_core_ct_uint_table_lookup(Wselect, Wtable,
                                               AN_limbs, welem, window);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
         // Only mark the codepath safe if there wasn't an unsafe codepath before
         if (mbedtls_mpi_optionally_safe_codepath != MBEDTLS_MPI_IS_PUBLIC) {
             mbedtls_mpi_optionally_safe_codepath = MBEDTLS_MPI_IS_SECRET;
diff --git a/library/bignum_core.h b/library/bignum_core.h
index ff6360b3e7..cf6485a148 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -817,7 +817,10 @@ void mbedtls_mpi_core_from_mont_rep(mbedtls_mpi_uint *X,
                                     mbedtls_mpi_uint mm,
                                     mbedtls_mpi_uint *T);
 
-#if defined(MBEDTLS_TEST_HOOKS)
+/*
+ * Can't define thread local variables with our abstraction layer: do nothing if threading is on.
+ */
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
 extern int mbedtls_mpi_optionally_safe_codepath;
 
 static inline void mbedtls_mpi_optionally_safe_codepath_reset(void)
diff --git a/tests/suites/test_suite_bignum_core.function b/tests/suites/test_suite_bignum_core.function
index 9c864ac870..ce663299d6 100644
--- a/tests/suites/test_suite_bignum_core.function
+++ b/tests/suites/test_suite_bignum_core.function
@@ -1232,22 +1232,22 @@ void mpi_core_exp_mod(char *input_N, char *input_A,
 
     /* Test the safe variant */
 
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     mbedtls_mpi_optionally_safe_codepath_reset();
 #endif
     mbedtls_mpi_core_exp_mod(Y, A, N, N_limbs, E, E_limbs, R2, T);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     TEST_EQUAL(mbedtls_mpi_optionally_safe_codepath, MBEDTLS_MPI_IS_SECRET);
 #endif
     TEST_EQUAL(0, memcmp(X, Y, N_limbs * sizeof(mbedtls_mpi_uint)));
 
     /* Test the unsafe variant */
 
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     mbedtls_mpi_optionally_safe_codepath_reset();
 #endif
     mbedtls_mpi_core_exp_mod_unsafe(Y, A, N, N_limbs, E, E_limbs, R2, T);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     TEST_EQUAL(mbedtls_mpi_optionally_safe_codepath, MBEDTLS_MPI_IS_PUBLIC);
 #endif
     TEST_EQUAL(0, memcmp(X, Y, N_limbs * sizeof(mbedtls_mpi_uint)));
@@ -1257,21 +1257,21 @@ void mpi_core_exp_mod(char *input_N, char *input_A,
     TEST_CALLOC(A_copy, A_limbs);
     memcpy(A_copy, A, sizeof(A_copy) * A_limbs);
 
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     mbedtls_mpi_optionally_safe_codepath_reset();
 #endif
     mbedtls_mpi_core_exp_mod(A, A, N, N_limbs, E, E_limbs, R2, T);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     TEST_EQUAL(mbedtls_mpi_optionally_safe_codepath, MBEDTLS_MPI_IS_SECRET);
 #endif
     TEST_EQUAL(0, memcmp(X, A, N_limbs * sizeof(mbedtls_mpi_uint)));
 
     memcpy(A, A_copy, sizeof(A) * A_limbs);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     mbedtls_mpi_optionally_safe_codepath_reset();
 #endif
     mbedtls_mpi_core_exp_mod_unsafe(A, A, N, N_limbs, E, E_limbs, R2, T);
-#if defined(MBEDTLS_TEST_HOOKS)
+#if defined(MBEDTLS_TEST_HOOKS) && !defined(MBEDTLS_THREADING_C)
     TEST_EQUAL(mbedtls_mpi_optionally_safe_codepath, MBEDTLS_MPI_IS_PUBLIC);
 #endif
     TEST_EQUAL(0, memcmp(X, A, N_limbs * sizeof(mbedtls_mpi_uint)));