diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 51c799f712f..0d018ff248b 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -715,7 +715,7 @@ class Registry implements DestructableInterface {
     ksort($suggestion_level);
     foreach ($suggestion_level as $level => $item) {
       foreach ($item as $preprocessor => $hook) {
-        if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
+        if (isset($cache[$hook]['preprocess functions']) && !in_array($preprocessor, $cache[$hook]['preprocess functions'])) {
           // Add missing preprocessor to existing hook.
           $cache[$hook]['preprocess functions'][] = $preprocessor;
         }