diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 0c1c25e..b09ef3b 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -605,45 +605,44 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
    */
   protected function completeSuggestion($hook, array &$cache) {
     $previous_hook = $hook;
-    $first_valid_hook = '';
+    $incomplete_previous_hook = array();
     // Continue looping if the candidate hook doesn't exist or if the candidate
     // hook has incomplete preprocess functions, and if the candidate hook is a
     // suggestion (has a double underscore).
-    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions'])) && $pos = strrpos($previous_hook, '__')) {
+    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
+      && $pos = strrpos($previous_hook, '__')) {
       // Find the first existing candidate hook that has incomplete preprocess
       // functions.
-      if (isset($cache[$previous_hook]) && !$first_valid_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
-        $first_valid_hook = $previous_hook;
+      if (isset($cache[$previous_hook]) && !$incomplete_previous_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $incomplete_previous_hook = $cache[$previous_hook];
+        unset($incomplete_previous_hook['incomplete preprocess functions']);
       }
       $previous_hook = substr($previous_hook, 0, $pos);
-      $this->mergePreprocessFunctions($hook, $previous_hook, $first_valid_hook, $cache);
+      $this->mergePreprocessFunctions($hook, $previous_hook, $incomplete_previous_hook, $cache);
     }
 
     // In addition to processing suggestions, include base hooks.
     if (isset($cache[$hook]['base hook']) && isset($cache[$cache[$hook]['base hook']])) {
       // Pass the current hook as the first valid hook if a base hook exists.
-      $this->mergePreprocessFunctions($hook, $cache[$hook]['base hook'], $hook, $cache);
+      $this->mergePreprocessFunctions($hook, $cache[$hook]['base hook'], $cache[$hook], $cache);
     }
-
-    // Remove the incompleteness flag.
-    unset($cache[$hook]['incomplete preprocess functions']);
   }
 
   /**
    * Merges the source hook's preprocess functions into the destination hook's.
    *
    * @param string $destination_hook_name
-   *   The name of the suggestion hook to complete.
+   *   The name of the hook to merge preprocess functions to.
    * @param string $source_hook_name
    *   The name of the hook to merge preprocess functions from.
-   * @param string $parent_hook_name
-   *   The name of the parent hook if it exists.
+   * @param array $parent_hook
+   *   The parent hook if it exists. Either an incomplete hook from suggestions
+   *   or a base hook.
    * @param array $cache
    *   The theme registry, as documented in
    *   \Drupal\Core\Theme\Registry::processExtension().
    */
-  protected function mergePreprocessFunctions($destination_hook_name, $source_hook_name, $parent_hook_name, array &$cache) {
-    $parent_hook = isset($cache[$parent_hook_name]['incomplete preprocess functions']) ? $cache[$parent_hook_name] : [];
+  protected function mergePreprocessFunctions($destination_hook_name, $source_hook_name, $parent_hook, array &$cache) {
     // If base hook exists clone of it for the preprocess function
     // without a template.
     // @see https://www.drupal.org/node/2457295