Toast推送通知服务空例外

Question

我正在尝试发送推送通知，但收到此错误：

  

类型＆＃39; System.ArgumentNullException＆＃39;的异常＆＃34; HttpWebRequest请求=（HttpWebRequest）WebRequest.Create（uri）;＆＃34;＆＃34;发生在System.dll中，但未在用户代码中处理）代码行...

这是我的发送通知代码：

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
        request.Method = WebRequestMethods.Http.Post;
        request.ContentType = "test/xml";
        request.ContentLength = message.Length;

        request.Headers.Add("X-MessageID", Guid.NewGuid().ToString());

        request.Headers["X-WindowsPhone-Target"] = "toast";
        request.Headers.Add("X-NotificationClass", "2");

        var requestStream = request.GetRequestStream();
        requestStream.Write(message, 0, message.Length);