The endpoint must be in Established state to perform this operation

Hi,

I am executing sample application subscribepresence in UCMA3.0 SDK.  The API remotePresenceView.StartSubscribingToPresentities throws the exception "The endpoint must be in Established state to perform this operation". Please advice me on this problem

try
{
     // Subscribe to target user.
      _target = new RemotePresentitySubscriptionTarget(_remoteUserUri);
       _remotePresenceView.StartSubscribingToPresentities(
         new RemotePresentitySubscriptionTarget[] { _target });

Thanks in advance.