Application crash wen i used LastTransferSpeed in OnTransferProgress Event

using System;

using System.Collections.Generic;

using System.Text;

using System.Windows.Forms;

using System.Threading;



namespace FTPExample

{

	public class FTPLib

	{

		MainForm m_Form;



		sfFTPLib.FTPConnectionMTAClass _ftp;



		private void OnTransferProgress(int nTransferBytesLo, int nTransferBytesHi)

		{

			Int64 nTransferBytes = (uint)nTransferBytesLo;

			nTransferBytes <<= 32;

			nTransferBytes += (uint)nTransferBytesHi;

			m_Form.label1.BeginInvoke(m_Form.m_DelegateShowTransferProgress, new Object[] { nTransferBytes.ToString() });

		}



		public FTPLib(string url, string username, string password, int port, MainForm form)

		{

			m_Form = form;



			sfFTPLib.FTPConnectionMTA _ftp = new sfFTPLib.FTPConnectionMTA();

			

			_ftp.OnTransferProgress += new sfFTPLib.IFTPConnectionEvents_OnTransferProgressEventHandler(OnTransferProgress);



			// Host 

			_ftp.Host = url;

			_ftp.Port = port;

			_ftp.Username = username;

			_ftp.Password = password;

			_ftp.Passive = true;

			_ftp.Timeout = 60000;

			...

		private void OnTransferProgress(int nTransferBytesLo, int nTransferBytesHi)

		{

			Int64 nTransferBytes = (uint)nTransferBytesLo;

			nTransferBytes <<= 32;

			nTransferBytes += (uint)nTransferBytesHi;

			int TransferSpeed = _ftp.LastTransferSpeed; // <---- crash here

			m_Form.label1.BeginInvoke(m_Form.m_DelegateShowTransferProgress, new Object[] { nTransferBytes.ToString() });

		}