diff -u b/core/modules/system/src/Tests/Theme/RegistryTest.php b/core/modules/system/src/Tests/Theme/RegistryTest.php
--- b/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -103,7 +103,7 @@
     $preprocess_functions = $registry_base_theme->get()['theme_test_function_suggestions']['preprocess functions'];
     $this->assertIdentical([
       'test_basetheme_preprocess_theme_test_function_suggestions',
-    ], $preprocess_functions);
+    ], $preprocess_functions, "Theme functions doesn't have template preprocess function");
   }
 
   /**