New Post: Can't tweet ...

Hi,
I discovered this great library but I can't use it to tweet.
My simple test is the following :

TwitterCredentials.SetCredentials(
                "accessToken",
                "accessTokenSecret",
                "APIkey",
                " APISecret");

            var tweet = Tweet.PublishTweet("Hello");

The token has all permissions but the tweet variable is always null !

Any ideas ?

Paolo.