diff --git a/src/AliasCleaner.php b/src/AliasCleaner.php
index 340a266..fab7c71 100644
--- a/src/AliasCleaner.php
+++ b/src/AliasCleaner.php
@@ -155,7 +155,7 @@ class AliasCleaner implements AliasCleanerInterface {
   /**
    * {@inheritdoc}
    */
-  public function cleanString($string, array $options = array()) {
+  public function cleanString($string = NULL, array $options = array()) {
     if (empty($this->cleanStringCache)) {
       // Generate and cache variables used in this method.
       $config = $this->configFactory->get('pathauto.settings');