diff --git a/lib/PayPal/Core/PPHttpConfig.php b/lib/PayPal/Core/PPHttpConfig.php
index d5cf4d2..81ff0ee 100644
--- a/lib/PayPal/Core/PPHttpConfig.php
+++ b/lib/PayPal/Core/PPHttpConfig.php
@@ -217,7 +217,7 @@ public function setUserAgent($userAgentString)
      *
      * @return array
      */
-    public function getHttpConstantsFromConfigs($configs = array(), $prefix)
+    public function getHttpConstantsFromConfigs($configs = array(), $prefix = null)
     {
         $arr = array();
         if ($prefix != null && is_array($configs)) {