diff --git c/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php w/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
index f94e6c5..26054fa 100644
--- c/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
+++ w/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
@@ -55,9 +55,7 @@ public function testInfoStylesheets() {
    *   A render array containing a theme override.
    */
   public function testTemplate() {
-    return array(
-      '#theme' => 'theme_test_template_test',
-    );
+      return theme('theme_test_template_test');
   }
 
   /**