用ASP.NET写的下载功能出问题了。。

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.IO;

public partial class 下载页面 : System.Web.UI.Page
{
  protected void Page_Load(object sender, EventArgs e)
  {
  string path =Request.QueryString["WordUrl"];  
  FileInfo file = new FileInfo(path);
  FileStream myfileStream = new FileStream(path, FileMode.Open, FileAccess.Read);
  byte[] filedata = new Byte[file.Length];
  myfileStream.Read(filedata, 0, (int)(file.Length));
  myfileStream.Close();
  Response.Clear();
  Response.ContentType = "application/msword";
  Response.AddHeader("Content-Disposition", "attachment;filename=shiti.doc");
  Response.Flush();
  Response.BinaryWrite(filedata);
  Response.End();

  }
}
这是我用ASP.NET写的下载代码。。。但却不能下载出现的问题提示是这样：
未处理 System.NullReferenceException
  Message="未将对象引用设置到对象的实例。"
  Source="WebDev.WebHos