ciao ragazzi sn un pivello di aspnet e sto studiando un po mi si è preentato un problema:ecco il codice che mi da un errore js

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form name="form1" id="form1" runat="server">
<asp:TextBox ID="txt1" runat="server"></asp:TextBox>

<asp:TextBox ID="Txt2" runat="server"></asp:TextBox>


<asp:Button ID="Cmd1" runat="server" Text="Login"/>
</form>
</body>
</html>


file vb
Partial Class _Default
Inherits System.Web.UI.Page
Protected Sub Cmd1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Cmd1.Click
Dim js As String
js = "alert(document.forms[0]['txt1'].value);"
Page.ClientScript.RegisterClientScriptBlock(Cmd1.G etType(), "js", js, True)


End Sub
End Class

l'erroe dice document.forms[0][txt1].value nullo
aiutatemi grazie....