[ASP.NET2.0] How to invoke a javascript inside layouttemplate

[mattmao]

I created a customed layout template in which contains a TextBox and Button.

<layouttemplate>
    <asp:TextBox id="TxtUser" runat="server" />
    <asp:Button id="myButton" runat="server" OnClientClick="myJavascriptFunction" />
</layouttemplate>

The javascript looks like this:

<script type=text/javascript>
function myJavascriptFunction ( )
{
var myVar = document.getElementById("<%= TxtUser.ClientID%>").value ;

...code goes on
}
</script>

But no matter where I put my javascript file, this script would always generate an error : Cannot find reference "TxtUser" in context.

My code works fine if I remove the layouttemplate, it just seems that the "TxtUser" id is "hidden" by the layouttemplate.

Can anyone help me? Thanks in advance!