Fix missing guard on XTS function in tests
diff --git a/tests/suites/test_suite_aes.function b/tests/suites/test_suite_aes.function
index 1832f2d..1315650 100644
--- a/tests/suites/test_suite_aes.function
+++ b/tests/suites/test_suite_aes.function
@@ -402,7 +402,9 @@
 {
     /* These calls accept NULL */
     TEST_VALID_PARAM( mbedtls_aes_free( NULL ) );
+#if defined(MBEDTLS_CIPHER_MODE_XTS)
     TEST_VALID_PARAM( mbedtls_aes_xts_free( NULL ) );
+#endif
 }
 /* END_CASE */