diff --git a/lib/yrewrite/seo.php b/lib/yrewrite/seo.php
index 0f057ac..a9df455 100644
--- a/lib/yrewrite/seo.php
+++ b/lib/yrewrite/seo.php
@@ -220,7 +220,7 @@ public function getHreflangTags()
 
     public function cleanString($str)
     {
-        return str_replace(["\n", "\r"], [' ', ''], $str);
+        return str_replace(["\n", "\r"], [' ', ''], ($str ?? ''));
     }
 
     // ----- global static functions