diff -u b/core/modules/system/src/Tests/Theme/RegistryTest.php b/core/modules/system/src/Tests/Theme/RegistryTest.php
--- b/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -31,6 +31,7 @@
    * Tests the behavior of the theme registry class.
    */
   function testRaceCondition() {
+
     // The theme registry is not marked as persistable in case we don't have a
     // proper request.
     \Drupal::request()->setMethod('GET');
@@ -39,9 +40,10 @@
     // entry to be written in __construct().
     $cache = \Drupal::cache();
     $lock_backend = \Drupal::lock();
-    $cid = 'theme_registry:runtime:core';
-    $registry = new ThemeRegistry($cache, $lock_backend, array('theme_registry'), $this->container->get('module_handler')->isLoaded());
+    $theme = \Drupal::theme();
+    $registry = new ThemeRegistry($cache, $lock_backend, $theme->getActiveTheme(), array('theme_registry'), $this->container->get('module_handler')->isLoaded());
 
+    $cid = 'theme_registry:runtime:' . $theme->getActiveTheme()->getName();
     $this->assertFalse(\Drupal::cache()->get($cid), 'Cache entry was not created by default.');
 
     // Trigger a cache miss for an offset.
@@ -61,7 +63,7 @@
     // Create a new instance of the class. Confirm that both the offset
     // requested previously, and one that has not yet been requested are both
     // available.
-    $registry = new ThemeRegistry($cache, $lock_backend, array('theme_registry'), $this->container->get('module_handler')->isLoaded());
+    $registry = new ThemeRegistry($cache, $lock_backend, $theme->getActiveTheme(), array('theme_registry'), $this->container->get('module_handler')->isLoaded());
     $this->assertTrue($registry->get('theme_test_template_test'), 'Offset was returned correctly from the theme registry');
     $this->assertTrue($registry->get('theme_test_template_test_2'), 'Offset was returned correctly from the theme registry');
   }