diff -u b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
--- b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -35,13 +35,9 @@
     /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
     $theme_manager = $this->container->get('theme.manager');
 
+    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme'));
     /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
     $library_discovery = $this->container->get('library.discovery');
-
-    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('classy'));
-    $this->assertFalse($library_discovery->getLibraryByName('test_theme', 'kitten'));
-
-    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme'));
     $this->assertTrue($library_discovery->getLibraryByName('test_theme', 'kitten'));
   }