diff --git a/app/code/core/Mage/Catalog/Helper/Product/Url.php b/app/code/core/Mage/Catalog/Helper/Product/Url.php
index 45f72855853..428f8a537ec 100644
--- a/app/code/core/Mage/Catalog/Helper/Product/Url.php
+++ b/app/code/core/Mage/Catalog/Helper/Product/Url.php
@@ -115,6 +115,6 @@ public function getConvertTable()
      */
     public function format($string)
     {
-        return strtr($string, $this->getConvertTable());
+        return $string === null ? '' : strtr($string, $this->getConvertTable());
     }
 }