问fileupload控制导致服务器错误

EN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}
protected void Button1_Click(object sender, EventArgs e)
{
    String fn = FileUpload1.FileName;
    string fp = Server.MapPath("images");
    if (fp.EndsWith("\\") == false)
    {
        fp = fp + "\\";
    }
    fp = fp + fn;
    FileUpload1.PostedFile.SaveAs(fp);
}
}