diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index 240aea9..8232cb0 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -808,7 +808,7 @@ protected function assertNoTitle($title, $message = '', $group = 'Other') {
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertThemeOutput($callback, array $variables = array(), $expected = '', $message = '', $group = 'Other') {
-    $output = \Drupal::theme()->render($callback, $variables);
+    $output = (string) \Drupal::theme()->render($callback, $variables);
     $this->verbose(
       '<hr />' . 'Result:' . '<pre>' . SafeMarkup::checkPlain(var_export($output, TRUE)) . '</pre>'
       . '<hr />' . 'Expected:' . '<pre>' . SafeMarkup::checkPlain(var_export($expected, TRUE)) . '</pre>'