OAuthException : Error validating access token: The session is invalid because the user logged out.

I have one facebook integrated application. After login ,if i am trying to logout the facebook. I am getting the Error in the bellow bolded section

if (Api != null) { Facebook_Graph_Toolkit.GraphApi.User U = new Facebook_Graph_Toolkit.GraphApi.User("me", Api.AccessToken);

AccessToken = Api.AccessToken;

UserId = U.ID; UserName = U.Name;

Email = U.Email; SmallPic = Api.GetPictureURL(U.ID);

ImgButtonWish.Enabled = true;

ImgButtonJourney.Enabled = true;