Dynamically creating a CollapsiblePanel

If dtmain.Rows.Count > 0 Then
            For i As Integer = 0 To dtmain.Rows.Count - 1

                Dim Main As New AjaxControlToolkit.CollapsiblePanelExtender
                Main.ID = "cp_" + i.ToString
                Main.CollapseControlID = "hp_" + i.ToString
                Main.CollapsedImage = "expand.gif "
                Main.ExpandControlID = "hp_" + i.ToString
                Main.ExpandedImage = "collapse.gif"
                Main.ImageControlID = "img" + i.ToString
                Main.TargetControlID = "Panel_" + i.ToString
                Main.Collapsed = True
                Me.up_menu.ContentTemplateContainer.Controls.Add(Main)

                Dim item As New MenuItem(dtmain.Rows(i)("group").ToString, "")
                str += "<table cellpadding='0' cellspacing='0' style='width: 279px'><tr><td style='width: 279px'>"
                str += "<div  runat="server"><asp:HyperLink ID='hp_" + i.ToString + "' runat='server' CssClass='handpoint' Font-Bold='True' Font-Names='Arial' Font-Size='10pt' Font-Underline='False' ForeColor='#333333' Width='205px'>" + dtmain.Rows(i)("group").ToString + "</asp:HyperLink>"
                str += "<asp:Image ID='img" + i.ToString + "' runat='server' ImageAlign='AbsMiddle' ImageUrl='~/collapse.gif' /></div></td></tr>"
                str += "<tr>"
                str += "<td style='width: 279px'>"
                str += "<asp:Panel ID='Panel_" + i.ToString + "' runat='server'>"
                str += "<table class='sampleStyleN' style='width: 265px'>"
                str += "<tr>"
                str += "<td style='width: 100px; height: 4px'>"
                str += "</td>"
                str += "</tr>"

and the rest of code follows, everything works fine, but the only problem is when i run the project, it says "The collapsible panel cannot find the TargetControlID"

Kindly help. Thank you