Why does a ModalPopupExtender fail when using SSL?

Question

I have created a modal popup using the ModalPopupExtender in Microsoft's AJAX 1.0 for .NET 2.0. It works great when the page doesn't isn't being accessed through SSL (http://) however the link to close the popup fails to fire if accessing the page through https://.

Is the ModalPopupExtender at blame? Is it a "Feature" of SSL to block popups, or is it something else I haven't thought of?

Here is the code I am using:

<asp:Button ID="btnHelp" runat="server" Text="?" CausesValidation="False" />

<asp:Panel ID="pnlHelp" BackColor="white" runat="server">

<asp:LinkButton ID="lnkClosePanel" runat="server" CausesValidation="False" OnClick="lnkCloseHelp_Click">Close</asp:LinkButton>
<p>Some Text</p>
</asp:Panel>

<cc1:ModalPopupExtender ID="popExt" runat="server" TargetControlID="btnHelp" PopupControlID="pnlHelp"></cc1:ModalPopupExtender>