authorization error using linqtotwitter for streaming api – Twitter API

i’m trying to get a stream of tweets using LinqToTwitter library and the below c# code, but i get this error:

Error 401 Unauthorized

public static SingleUserAuthorizer auth;
    static void Main(string[] args)
    {
        Task task = new Task(getStreamOfTweets);
        task.Start();
        task.Wait();
        Console.ReadLine();
    }

    static async void getStreamOfTweets()
    {
        auth = new SingleUserAuthorizer
        {
            CredentialStore = new SingleUserInMemoryCredentialStore
            {
                ConsumerKey = CUSTOMER_KEY,
                ConsumerSecret = CUSTOMER_SECRET,
                AccessToken = ACCESS_TOKEN,
                AccessTokenSecret = ACCESS_TOKEN_SECRET
            }
        };
        var context = new TwitterContext(auth);
        int count = 0;
        await (from strm in context.Streaming
               where strm.Type == StreamingType.Filter
               && strm.Track == "federer"
               select strm)
            .StartAsync(async strm =>
            {
                string message =
                    string.IsNullOrEmpty(strm.Content) ?
                        "Keep-Alive" : strm.Content;
                Console.WriteLine(
                    (count + 1).ToString() +
                    ". " + DateTime.Now +
                    ": " + message + "n");

                if (count++ == 5)
                    strm.CloseStream();
            });
    }

notes:

• the permission in twitter app is “Read, Write and Access direct messages”

• i can get tweet by REST API correctly