diff --git a/tests/src/bignum_helpers.c b/tests/src/bignum_helpers.c
index 4dd3791..efb2eca 100644
--- a/tests/src/bignum_helpers.c
+++ b/tests/src/bignum_helpers.c
@@ -99,7 +99,18 @@
     if (ret != 0) {
         return ret;
     }
-    ret = mbedtls_mpi_mod_modulus_setup(N, p, limbs, int_rep);
+
+    switch (int_rep) {
+        case MBEDTLS_MPI_MOD_REP_MONTGOMERY:
+            ret = mbedtls_mpi_mod_modulus_setup(N, p, limbs);
+            break;
+        case MBEDTLS_MPI_MOD_REP_OPT_RED:
+            ret = mbedtls_mpi_mod_optred_modulus_setup(N, p, limbs, NULL);
+            break;
+        default:
+            ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+            break;
+    }
     if (ret != 0) {
         mbedtls_free(p);
     }