diff --git a/core/tests/Drupal/KernelTests/Core/Asset/AttachedAssetsTest.php b/core/tests/Drupal/KernelTests/Core/Asset/AttachedAssetsTest.php
index 76445fe..72e7b6f 100644
--- a/core/tests/Drupal/KernelTests/Core/Asset/AttachedAssetsTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Asset/AttachedAssetsTest.php
@@ -195,7 +195,8 @@ function testSettings() {
 
     $js = $this->assetResolver->getJsAssets($assets, FALSE)[1];
     $js_render_array = \Drupal::service('asset.js.collection_renderer')->render($js);
-    $rendered_js = $this->renderer->renderPlain($js_render_array);
+    // Cast to string since this returns a \Drupal\Core\Render\Markup object.
+    $rendered_js = (string) $this->renderer->renderPlain($js_render_array);
 
     // Parse the generated drupalSettings <script> back to a PHP representation.
     $startToken = '{';
@@ -204,7 +205,7 @@ function testSettings() {
     $end = strrpos($rendered_js, $endToken);
     // Convert to a string, as $renderer_js is a \Drupal\Core\Render\Markup
     // object.
-    $json  = Unicode::substr((string) $rendered_js, $start, $end - $start + 1);
+    $json  = Unicode::substr($rendered_js, $start, $end - $start + 1);
     $parsed_settings = Json::decode($json);
 
     // Test whether the settings for core/drupalSettings are available.