diff --git a/webapp/controller/ShopController.class.php b/webapp/controller/ShopController.class.php
index 371a1ad6..12c501fe 100644
--- a/webapp/controller/ShopController.class.php
+++ b/webapp/controller/ShopController.class.php
@@ -420,7 +420,7 @@ class ShopController extends BaseAuthedController
         $arrCosts = explode('|', $costs['param_value']);
 
         $max_count = count($arrCosts);
-        $cost = $count < $max_count ? $arrCosts[$count] : null;
+        $cost = $count < $max_count ? $arrCosts[$count] : -1;
 
         $this->_rspData(
             array(