diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index a0af702..d38599d 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -199,6 +199,7 @@ public function setThemeManager(ThemeManagerInterface $theme_manager) {
    */
   protected function init($theme_name = NULL) {
     if ($this->initialized) {
+      throw new \Exception('initialized');
       return;
     }
     // Unless instantiated for a specific theme, use globals.