diff --git a/core/modules/system/src/Tests/Theme/RegistryTest.php b/core/modules/system/src/Tests/Theme/RegistryTest.php
index c6e6347..d03b52d 100644
--- a/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -102,8 +102,9 @@ public function testMultipleSubThemes() {
 
     $preprocess_functions = $registry_base_theme->get()['theme_test_function_suggestions']['preprocess functions'];
     $this->assertIdentical([
+      'template_preprocess_theme_test_function_suggestions',
       'test_basetheme_preprocess_theme_test_function_suggestions',
-    ], $preprocess_functions, "Theme functions doesn't have template preprocess function");
+    ], $preprocess_functions, "Theme functions don't have template_preprocess but do have template_preprocess_HOOK");
   }
 
   /**
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 7ba5c2d..686e411 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -76,6 +76,12 @@ function theme_test_page_bottom(array &$page_bottom) {
 }
 
 /**
+ * Implements template_preprocess_HOOK() for theme_test_function_suggestions theme functions.
+ */
+function template_preprocess_theme_test_function_suggestions(&$variables) {
+}
+
+/**
  * Theme function for testing _theme('theme_test_foo').
  */
 function theme_theme_test_foo($variables) {