diff --git c/core/modules/system/tests/modules/common_test/lib/Drupal/common_test/Controller/CommonTestController.php w/core/modules/system/tests/modules/common_test/lib/Drupal/common_test/Controller/CommonTestController.php
index bf155ab..661b37a 100644
--- c/core/modules/system/tests/modules/common_test/lib/Drupal/common_test/Controller/CommonTestController.php
+++ w/core/modules/system/tests/modules/common_test/lib/Drupal/common_test/Controller/CommonTestController.php
@@ -67,12 +67,10 @@ public function typeLinkActiveClass() {
    */
   public function drupalRenderInvalidKeys() {
     define('SIMPLETEST_COLLECT_ERRORS', FALSE);
-
     // Keys that begin with # may contain a value of any type, otherwise they must
     // contain arrays.
     $element = array('child' => 'This should be an array.');
-
-    return $element;
+    return drupal_render($element);
   }
 
 }