diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index d6624cc..457fef1 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -289,7 +289,7 @@ protected function theme($hook, $variables = array()) {
 
       // If base hook has variables defined we want to merge them so they don't
       // have to be set manually when base hook is defined.
-      if (!empty($base_hook_info['variables'])) {
+      if (!empty($base_hook_info['variables']) && isset($variables['variables'])) {
         $variables['variables'] = array_merge($base_hook_info['variables'], $variables['variables']);
       }