diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index a21ad47..2bf8024 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -384,13 +384,20 @@
         TEST_ASSERT(mbedtls_pk_get_type(&issuer_key) == MBEDTLS_PK_OPAQUE);
     }
 
+#if !defined(MBEDTLS_BIGNUM_C)
     TEST_ASSERT(mbedtls_test_read_mpi(&serial, serial_str) == 0);
+#endif
 
     if (ver != -1) {
         mbedtls_x509write_crt_set_version(&crt, ver);
     }
 
+#if !defined(MBEDTLS_BIGNUM_C)
     TEST_ASSERT(mbedtls_x509write_crt_set_serial(&crt, &serial) == 0);
+#else
+    TEST_ASSERT(mbedtls_x509write_crt_set_serial_new(&crt, serial_str,
+                                                     strlen(serial_str)) == 0);
+#endif
     TEST_ASSERT(mbedtls_x509write_crt_set_validity(&crt, not_before,
                                                    not_after) == 0);
     mbedtls_x509write_crt_set_md_alg(&crt, md_type);