New Post: Tweet.GetTweet returns null

Passing in a valid Tweet.Id into GetTweet(), for example

                    var tweet = Tweet.GetTweet (theTweetId);

always returns null and the following exception is lodged:

TwitterDescription "Not Found - The URI requested is invalid or the resource requested, such as a user, does not exists. Also returned when the requested format is not supported by the requested method."

I have even trapped a Tweet sent by another user in UserStream/BackgroundWorkerOnDoWork routine and passed its Tweet.Id straight into this method and it still returns null

Are there any common causes of this ?

Thanks