diff -u b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
--- b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
@@ -33,3 +33,3 @@
  */
-function test_basetheme_preprocess_theme_test_function_suggestions($variables) {
+function test_basetheme_preprocess_theme_test_function_suggestions(&$variables) {
 }