Error when using an AJAX Calendar Extender in UpdatePanel

On my webpage, i have an UpdatePanel that contains an asp:panel Panel and some controls with an Ajax CalendarExtender.  The Panel initially has visibility set to false.  When i select a value from a dropdown, a GridView loads and if the number of rows in the gridview are greater than 0, i set the panel visibility property to true and call the Update method on the UpdatePanel.  When I do this, I receive the following js error:

0x800a139e - JavaScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: Extender control 'tbIDMdate_CalendarExtender' is not a registered extender control. Extender controls must be registered using RegisterExtenderControl() before calling RegisterScriptDescriptors().
Parameter name: extenderControl

I have done a bunch of research and so far nothing has worked.  I'm not sure what to do to get it to work, the error appears when I try to set the visibility property of the Panel to True.  My toolscriptmanager is located in the master page.

<asp:UpdatePanelID="updPnlAssignDeliveryMethods"UpdateMode="Conditional"runat="server">
                                <ContentTemplate>
                                    <asp:PanelID="pnlAssignDeliveryMethods"runat="server"Visible="false">
                                        <asp:LabelID="Label18"runat="server"Text="IDM Date:"></asp:Label>
                                        <asp:TextBoxID="tbIDMdate"runat="server"Width="75px"></asp:TextBox>
                                            <ajaxToolkit:CalendarExtenderID="tbIDMdate_CalendarExtender"runat="server"Enabled="True"TargetControlID="tbIDMdate">
                                            </ajaxToolkit:CalendarExtender>
                                        <asp:LabelID="Label19"runat="server"Text="Delivery Method:"></asp:Label>
                                        <asp:ButtonID="btnAssignSD"runat="server"CssClass="buttonBlue"OnClick="btnAssignSD_Click"Text="Seat Delivery"/>
                                        &nbsp;<asp:ButtonID="btnAssignSelfDeploy"runat="server"CssClass="buttonBlue"OnClick="btnAssignSelfDeploy_Click"Text="Self Deploy"/>
                                        &nbsp;<asp:ButtonID="btnAssignFS"runat="server"CssClass="buttonBlue"OnClick="btnAssignFS_Click"Text="Field Services"/>
                           
                                    </asp:Panel>
                                </ContentTemplate>
                            </asp:UpdatePanel>