diff -u b/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
--- b/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -290,7 +290,7 @@
         // Set a variable for the 'theme_hook_suggestion'. This is used to
         // maintain backwards compatibility with template engines.
         $theme_hook_suggestion = $hook;
-        $info['preprocess functions'] += $base_hook_info['preprocess functions'];
+        $info['preprocess functions'] = $base_hook_info['preprocess functions'] + $info['preprocess functions'];
       }
     }
     if (isset($info['preprocess functions'])) {