diff --git a/core/modules/simpletest/src/Form/SimpletestTestForm.php b/core/modules/simpletest/src/Form/SimpletestTestForm.php
index f6d0367..aadc4a9 100644
--- a/core/modules/simpletest/src/Form/SimpletestTestForm.php
+++ b/core/modules/simpletest/src/Form/SimpletestTestForm.php
@@ -127,8 +127,8 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#suffix' => '<a href="#" class="simpletest-collapse">(' . $this->t('Collapse') . ')</a>',
     );
     $form['tests']['#attached']['drupalSettings']['simpleTest']['images'] = [
-      $this->renderer->renderPlain($image_collapsed),
-      $this->renderer->renderPlain($image_extended),
+      (string) $this->renderer->renderPlain($image_collapsed),
+      (string) $this->renderer->renderPlain($image_extended),
     ];
 
     // Generate the list of tests arranged by group.