error occur

 hi all
i got an error like this
InvalidOperationException was unhandled by user code
A form tag with runat=server must exist on the Page to use SetFocus() or the Focus property.
any one please help , 
my code is :
protected void Page_Load(object sender, EventArgs e)
    {
        this.lgnUsers.Focus();
    }
 
thanks
gandee 
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