Command webbrowser

Question

I can not fix the error anyone can help me? ..

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;


namespace test
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }


        private void button1_Click(object sender, EventArgs e)
        {


            webBrowser1.Navigate(" http://www.httpsurf.com/ ");


    //the debugger tells me that the error in this line    
          webBrowser1.Document.GetElementById("url").InnerText = textBox1.Text;


            webBrowser1.Document.GetElementById("go").InvokeMember("Click");
       
        }


        private void textBox1_TextChanged(object sender, EventArgs e)
        {


        }
    }
}