diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index f3a161c..6893c8b 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -669,6 +669,11 @@
     TEST_LE_S(1, ret);
     TEST_ASSERT(strcmp((char *) out, parsed_name) == 0);
 
+    /* Check that calling a 2nd time with the same param (now non-NULL)
+     * returns an error as expected. */
+    ret = mbedtls_x509_string_to_names(&names, name);
+    TEST_EQUAL(ret, MBEDTLS_ERR_X509_BAD_INPUT_DATA);
+
 exit:
     mbedtls_asn1_free_named_data_list(&names);