getting ftp 505 error

private bool GetFileSize(FileInfo targetFI, long FileUploadedSize, string g_Subdivision,TMWaitDialog tmwaitdialog)
          {
              FtpWebRequest reqFTP;
              long fileSize = 0;
              try
              {
                  string Uri = string.Empty;
                  Uri = @"ftp://" + ftpServerIP + "/" + g_Subdivision + "/" + targetFI.Name.ToUpper().Trim();
                  reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri(Uri.Trim()));
                  reqFTP.Method = WebRequestMethods.Ftp.GetFileSize;
                  reqFTP.UseBinary = true;
                  reqFTP.KeepAlive = false;
                  reqFTP.Credentials = new NetworkCredential(ftpUserID, ftpPassword);
                  FtpWebResponse response = (FtpWebResponse)reqFTP.GetResponse();
                  Stream ftpStream = response.GetResponseStream();
                  fileSize = response.ContentLength;
                  
                  ftpStream.Close();
                  response.Close();
                return (fileSize == FileUploadedSize);
              }
              catch (Exception ex)
              {
                  MessageBox.Show(ex.Message);
                  return false;
              }
             
          }