I have a Button inside an UpdatePanel. The button is being used as the OK button for a ModalPopupExtender. For some reason, the button click event is not firing. Any ideas? Am I missing something?
<asp:updatepanel id="UpdatePanel1" runat="server">
    <ContentTemplate>
        <cc1:ModalPopupExtender ID="ModalDialog" runat="server" 
            TargetControlID="OpenDialogLinkButton"
            PopupControlID="ModalDialogPanel" OkControlID="ModalOKButton"
            BackgroundCssClass="ModalBackground">
        </cc1:ModalPopupExtender>
        <asp:Panel ID="ModalDialogPanel" CssClass="ModalPopup" runat="server">
            ...
            <asp:Button ID="ModalOKButton" runat="server" Text="OK" 
                        onclick="ModalOKButton_Click" />
        </asp:Panel>
    </ContentTemplate>
</asp:updatepanel>