The endpoint /_vti_bin/xxx.svc is not accessible in the context of a SharePoint App

Hi,
I develop custom oData service under ISAPI folder and develop app to consume that service. And i got the following error message:
The endpoint http://app-15a87dc7e1cd9a.spdemoapp.com/_vti_bin/xxx.svc is not accessible in the context of a SharePoint App.
If I access the service from host web url, it is working fine. Mean that service is working fine.
Which setting did i miss out ? 

kmhsad