diff --git a/core/modules/field/tests/modules/field_test/field_test.module b/core/modules/field/tests/modules/field_test/field_test.module
index c7e3b1c..215f9cc 100644
--- a/core/modules/field/tests/modules/field_test/field_test.module
+++ b/core/modules/field/tests/modules/field_test/field_test.module
@@ -272,7 +272,8 @@ function field_test_field_formatter_settings_form_alter(&$element, &$form_state,
  * Implements hook_field_formatter_settings_summary_alter().
  */
 function field_test_field_formatter_settings_summary_alter(&$summary, $context) {
-  $summary .= '<br />field_test_field_formatter_settings_summary_alter';
+  $summary[] = '<br />field_test_field_formatter_settings_summary_alter';
+  return $summary;
 }
 
 /**