diff --git a/src/UnsavedIndexConfiguration.php b/src/UnsavedIndexConfiguration.php
index 5b6e4a4..a56f2bf 100644
--- a/src/UnsavedIndexConfiguration.php
+++ b/src/UnsavedIndexConfiguration.php
@@ -302,7 +302,7 @@ class UnsavedIndexConfiguration implements IndexInterface, UnsavedConfigurationI
   /**
    * {@inheritdoc}
    */
-  public function preprocessIndexItems(array &$items) {
+  public function preprocessIndexItems(array $items) {
     return $this->entity->preprocessIndexItems($items);
   }