diff --git a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
index 9e3aaa7..44b9724 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
@@ -434,7 +434,9 @@ public function testScalarLazybuilderCallbackContext() {
       'null' => NULL,
     ]];
 
-    $this->renderer->renderRoot($element);
+    $result = $this->renderer->renderRoot($element);
+    $this->assertInstanceOf('\Drupal\Core\Render\SafeString', $result);
+    $this->assertEquals('<p>This is a rendered placeholder!</p>', (string) $result);
   }
 
   /**