NUnitAsp System.InvalidOperationException Error

[nirupama]

Sir,

I am new to the Nunit,nunitasp i am gettin an error as
"System.InvalidOperationException:the specified node cannot be inserted as the valid child of this node, because the specified node is the wrong type"
i have used nunit.framework,nunitasp as references ,but when i am using txt1.tex="asdf", then i am getting can u plz send the reply as soon as possible

 ****************** 
using System;
using System.Web;
 
using MyMathProject;
using MyWebApp;
 
using NUnit.Extensions;
using NUnit.Extensions.Asp;
using NUnit.Extensions.Asp.AspTester ;
using NUnit.Extensions.Asp.HtmlTester;
namespace MyMathProjectTest
{
    /// <summary>
    /// Summary description for Class1.
    /// </summary>
    /// 
    using NUnit.Framework;
    [TestFixture]
 
    public class Class1: NUnit.Extensions.Asp.WebFormTestCase
    {
 
 
        public Class1()
        {
            //
            // TODO: Add constructor logic here
            //
        }
        [Test] public void Subtract() 
        { 
             //Assert.AreEqual(1.1, MyMathProject.MySimpleMath.Subtract(2.2, 1.1), "Subtraction failed"); 
        } 
 
        [Test] public void formLayout()
        {
            NUnit.Extensions.Asp.AspTester.TextBoxTester txt1 = new TextBoxTester("TextBox1", CurrentWebForm); 
            TextBoxTester txt2 = new TextBoxTester("TextBox2", CurrentWebForm); 
            ButtonTester but1 = new ButtonTester("Button1", CurrentWebForm); 
            Browser.GetPage("http://localhost/MyWebApp/Custtest.aspx");
            txt1.Text="asdf";
        }
    }
 
}