diff --git a/src/Subscriber.php b/src/Subscriber.php
index b661f69..ddded92 100644
--- a/src/Subscriber.php
+++ b/src/Subscriber.php
@@ -181,7 +181,7 @@ private function change_subscription($mode, $topic_url, $http_function = false)
      *
      * @return mixed
      */
-    private function http($url, $post_string)
+    private function http($url, $post_string = false)
     {
 
         // add any additional curl options here