diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index a0af702..7c9f4d1 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Core\Theme;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\DestructableInterface;
@@ -568,7 +569,7 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
       }
 
       // Merge the newly created theme hooks into the existing cache.
-      $cache = $result + $cache;
+      $cache = NestedArray::mergeDeep($cache, $result);
     }
 
     // Let themes have variable preprocessors even if they didn't register a