Error in request.GetResponse()

string parameters = @"OrderId,OrderedDate,OrderType,ProductId,SubscriptionPeriod,SubscriptionPrice,SubscriptionVolumeCount,SubscriptionScheduleId,UserId" +
                Environment.NewLine + "1,2013-08-07 19:41:28,1,1300002727,1,800,1,961193,1";

                   // string parameters = strArray[1];
                    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
                    request.ContentType = "application/x-www-form-urlencoded";
                    request.Method = "POST";
                    request.KeepAlive = false;

                    byte[] bytes = Encoding.ASCII.GetBytes(parameters);
                    request.ContentLength = bytes.Length;
                    using (Stream os = request.GetRequestStream()) // this line gets the error
                    {
                        os.Write(bytes, 0, bytes.Length);
                        os.Close();
                        using (WebResponse response = request.GetResponse())
                        {
                            if (response == null)
                            {
                                Console.WriteLine("Response is null");
                            }
                            using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                            {
                                string s=reader.ReadToEnd().Trim();
                                ErrorLog.WriteLog("FujiApiPost", "FujiApiPostfn", "result:" + s, string.Empty, string.Empty);

                            }
                        }
                    }