How can I have optional parameters in Symfony2 route?

I have this code below:

/**
 * Lists all User entities.
 *
 * @Route("/{cid}",defaults={"cid" = null},name="user")
 * @Template()
 */
public function indexAction($cid=null)
{}

Now if I type site/user/1 then it works, but if I type site/user/ it says:

No route found

How can I have it that both routes work?