(OAuthException - #2500)必须使用活动访问令牌来查询有关当前用户的信息

        string accessToken = GetAccessToken();
        string accessKey = accessToken.Split('=')[1];
        var client = new FacebookClient(accessKey);
        dynamic me = client.Get("me");

这是获取访问令牌的方法，它确实返回了有效的访问令牌

private static string GetAccessToken()
    {
        // Create a request using a URL that can receive a post. 
        WebRequest request = WebRequest.Create("https://graph.facebook.com/oauth/access_token?grant_type=client_credentials&client_id=201193246663533&client_secret=secretkeyhere");
        // Set the Method property of the request to POST.
        request.Method = "POST";
        // Create POST data and convert it to a byte array.
        string postData = "This is a test that posts this string to a Web server.";
        byte[] byteArray = Encoding.UTF8.GetBytes(postData);
        // Set the ContentType property of the WebRequest.
        request.ContentType = "application/x-www-form-urlencoded";
        // Set the ContentLength property of the WebRequest.
        request.ContentLength = byteArray.Length;
        // Get the request stream.
        Stream dataStream = request.GetRequestStream();
        // Write the data to the request stream.
        dataStream.Write(byteArray, 0, byteArray.Length);
        // Close the Stream object.
        dataStream.Close();
        // Get the response.
        WebResponse response = request.GetResponse();
        // Display the status.
        Console.WriteLine(((HttpWebResponse)response).StatusDescription);
        // Get the stream containing content returned by the server.
        dataStream = response.GetResponseStream();
        // Open the stream using a StreamReader for easy access.
        StreamReader reader = new StreamReader(dataStream);
        // Read the content.
        string responseFromServer = reader.ReadToEnd();
        // Display the content.
        Console.WriteLine(responseFromServer);
        // Clean up the streams.
        reader.Close();
        dataStream.Close();
        response.Close();
        return responseFromServer;
    }

但是，当我调试时

dynamic me = client.Get("me");

引发此异常：

（OAuthException - #2500） 必须使用活动访问令牌来查询有关当前用户的信息。

我该如何解决这个问题？