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
@@ -611,7 +611,6 @@
           $cache[$hook]['base hook'] = $previous_hook;
         }
       }
-      return $hook;
     }
   }
 
@@ -679,7 +678,7 @@
           // Process non-existing hook and register it.
           // Look for a previously defined hook that is either a less specific
           // suggestion hook or the base hook.
-          $hook = $this->completeSuggestion($hook, $cache);
+          $this->completeSuggestion($hook, $cache);
           $cache[$hook]['preprocess functions'][] = $preprocessor;
         }
       }
@@ -692,7 +691,7 @@
       // from a pattern. This is typically set from
       // drupal_find_theme_functions() and drupal_find_theme_templates().
       if (isset($info['incomplete preprocess functions'])) {
-        $hook = $this->completeSuggestion($hook, $cache);
+        $this->completeSuggestion($hook, $cache);
         unset($cache[$hook]['incomplete preprocess functions']);
       }