ConfirmButtonExtender with ModalPopupExtender : TargetControlID error

Question

I am receiving the following error when using a ConfirmButtonExtender with a ModalPopupExtender. The error occurs in AjaxControlToolkit.ConfirmButtonExtender.RegisterDisplayModalPopup(). I located the source to this method and visually stepped through it. I can't find anything wrong with the markup.

    <table cellpadding="0" cellspacing="0" border="0">
        <tr>               
            <td class="modalBody">     
                 <!-- Body text -->    
                 <asp:Label ID="lblMsg" runat="server" Text="" class="msg" />
            </td>
        </tr>
        <tr>
            <td align="center" class="modalFooter">
               <asp:Button ID="btnConfirm" runat="server" CausesValidation="false" 
                    OnClick="btnCancel_Click" Text="Cancel" OnClientClick="disableSubmit();return false;"  />

               <asp:Button ID="btnCancel" runat="server" CausesValidation="false" 
                    OnClick="btnCancel_Click" Text="Cancel" OnClientClick="disableSubmit();return false;"  />
            </td>                
        </tr>
    </table>
</asp:Panel>