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
@@ -636,7 +636,9 @@
         unset($cache[$hook]['preprocess functions']);
       }
       // Ensure uniqueness.
-      $cache[$hook]['preprocess functions'] = array_unique($cache[$hook]['preprocess functions']);
+      if (isset($cache[$hook]['preprocess functions'])) {
+        $cache[$hook]['preprocess functions'] = array_unique($cache[$hook]['preprocess functions']);
+      }
     }
   }
 
reverted:
--- b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
+++ /dev/null
@@ -1 +0,0 @@
-{{ foo }}
reverted:
--- b/core/themes/bartik/templates/theme-test-preprocess-suggestions--suggestion.html.twig
+++ /dev/null
@@ -1 +0,0 @@
-{{ foo }}