diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -603,7 +603,7 @@
       // @see https://www.drupal.org/node/2457295
       if (isset($cache[$previous_hook]) && !isset($cache[$previous_hook]['incomplete preprocess functions'])) {
         $cache[$hook] = $incomplete_previous_hook + $cache[$previous_hook];
-        if (!empty($incomplete_previous_hook['preprocess functions'])) {
+        if (isset($incomplete_previous_hook['preprocess functions']) && isset($cache[$previous_hook]['preprocess functions'])) {
           $diff = array_diff($incomplete_previous_hook['preprocess functions'], $cache[$previous_hook]['preprocess functions']);
           $cache[$hook]['preprocess functions'] = array_merge($cache[$previous_hook]['preprocess functions'], $diff);
         }