Add FilteredTextBoxExtender problem

I am trying to add a FilteredTextBoxExtender via code in a ListView and it is not working (not throwing error either).  My code is below. Am I missing something?  Thanks.

    Protected Sub lvSchedule_ItemDataBound(sender As Object, e As System.Web.UI.WebControls.ListViewItemEventArgs) Handles lvSchedule.ItemDataBound

                Dim fextender As AjaxControlToolkit.FilteredTextBoxExtender = New AjaxControlToolkit.FilteredTextBoxExtender
                Dim tb As TextBox = CType(lvSchedule.FindControl("txtCarbLiquid"), TextBox)
                fextender.FilterMode = AjaxControlToolkit.FilterModes.ValidChars
                fextender.ValidChars = "0123456789"
                fextender.TargetControlID = tb.ID

                lvSchedule.Controls.Add(fextender)

    END Sub