diff --git a/model/iamport-payment-callback.php b/model/iamport-payment-callback.php
index 96467e9..94c57f1 100644
--- a/model/iamport-payment-callback.php
+++ b/model/iamport-payment-callback.php
@@ -28,6 +28,7 @@ public function __construct($user_code, $api_key, $api_secret)
 
     public function handle_callback()
     {
+      global $wp;
       if ( ! empty( $_GET[self::QUERY_CALLBACK] ) ) {
         $wp->query_vars[self::QUERY_CALLBACK] = $_GET[self::QUERY_CALLBACK];
       }