子线程中的WebBrowser，遍历标记出错


using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;namespace 测试I
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }        private void Form1_Load(object sender, EventArgs e)
        {
            Thread t = new Thread(new ThreadStart(Open));
            t.Start();
        }        private void Open()
        {
            wb.Navigate("http://baidu.com");
            wb.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(wbe_Auto);
        }        private void wbe_Auto(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            try
            {
                HtmlElement tmp = wb.Document.All["kw"];                wb.Document.All["subject"].SetAttribute("value", "aaaa");
            }
            catch (Exception ex)
            {
                txb.Text = ex.ToString();
            }
        }    }
}System.InvalidCastException: 指定的转换无效。
   在 System.Windows.Forms.UnsafeNativeMethods.IHTMLDocument2.GetLocation()
   在 System.Windows.Forms.WebBrowser.get_Document()
   在 测试I.Form1.wbe_Auto(Object sender, WebBrowserDocumentCompletedEventArgs e) 位置 d:\My Documents\Visual Studio 2005\Projects\测试I\测试I\Form1.cs:行号 36
请问如何在子线程中使用WebBrowser遍历HTML标记呢？