QNetworkAccessManager doesn't work with VPN connection

I am developing a Qt based twitter client application, but in my country, access to twitter.com is blocked.
Then I setup a VPN in my symbian 3 phone, set the access point as default connection. now I can access twitter.com with
the phone's web browser. Everything works well till now.

But when I run my application, it sends out a HTTP request and can't get response from twitter.com. I have verified that,
if I use Symbian native HTTP engine, I can get response from the server with VPN connection.

The bug is that, QNetworkAccessManager can't work with VPN connection.