New Post: Get user Mentions with custom query

I am trying to get user Mentions with custom query. But this code

TwitterAccessor.ExecuteGETQuery<List<IMention>>(String.Format("https://api.twitter.com/1/statuses/mentions.json?since_id={0}",since.Id));

returns null, and exception in TwitterAccessor says "The remote server returned an error: (401) Unauthorized.".
I am authorized and Tweetinvi.User.GetLoggedUser().GetMentionsTimeline() works fine.

Please help.