diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
index ecd4d56..5ca1d0f 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
@@ -38,6 +38,9 @@ public function setUp() {
    * Ensures prepare hooks work and are invoked and in the correct order.
    */
   public function testThemePrepare() {
+    \Drupal::config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
     $this->drupalGet('theme-test/prepare');
 
     $this->assertRaw('<p>moduleVariable1: Drupal\theme_test\ThemeTestController::prepare<br>