SetFocus gridview control

[postings]

Hi

I'm trying to set the focus to a textbox situated in the footer of a
gridview, when the page is loading. (note textbox is in a template).

So far I've got:
-------------------------------
Protected Sub Page_Load(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Load

Page.SetFocus(gvPortfolio.FooterRow.FindControl("txtName"))

End Sub
-------------------------------

Unfortunately this comes up with yellow screen of death:

"System.InvalidOperationException: The DefaultButton of 'form1' must be
the ID of a control of type IButtonControl."

Any ideas?

Thanks

Alex