Setting focus to a text box on page load

All of a sudden code that had been working at my websites to set the focus
to a particular text box on page load now produces an error.

The code is:

'put the focus on the first text box
Dim strScript As String
strScript = "<script>"
strScript = strScript & "document.forms[0].txtUserName.focus();"
strScript = strScript & "</script>"
Page.RegisterStartupScript("ClientScript", strScript)

And the error produced is: 'document.forms.0.txtUserName' is null or not an
object

It is happening in my development computer as well at sites. All I can
figure is that some Windows update changed IIS in some way.

What is the solution?


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