关闭服务器协议,c# – 底层连接已关闭.服务器提交了协议违规

我试图使用以下代码获取FTPS FileZilla服务器的目录列表：

ftpRequest = (FtpWebRequest)FtpWebRequest.Create(host + "/" + directory);

ftpRequest.EnableSsl = true;

ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(ValidateCertificate);

ftpRequest.Credentials = new NetworkCredential(user,pass);

ftpRequest.UseBinary = true;

ftpRequest.UsePassive = true;

ftpRequest.KeepAlive = true;

ftpRequest.Method = WebRequestMethods.Ftp.ListDirectory;

ftpResponse = (FtpWebResponse)ftpRequest.GetResponse();

执行FtpWebResponse)ftpRequest.GetResponse()时出现异常：

the underlying connection was closed. The server committed a protocol

violation.

当我切换到正常的FTP连接.一切正常.

我错过了建立这个FTPS连接的东西吗？

感谢帮助