ValidatorCalloutExtender with a CustomValidator on a CheckBoxList

I have seen this question asked before but no working solutions have been provided.  I am trying to create a client side validation for my ASP.NET CheckBoxList control to make sure at least one item is selected.  Here is the code for the CheckBoxList:

<asp:CheckBoxList ID="Regions" runat="server"
    DataTextField="MY_TEXT" 
    DataValueField="MY_VALUE"
    RepeatColumns="5"
    RepeatLayout="Table"
    RepeatDirection="Vertical"
    ValidationGroup="ModalValidation"></asp:CheckBoxList><asp:CustomValidator ID="cvRegions" runat="server"
    ClientValidationFunction="ValidateCheckBoxList"
    ErrorMessage="Please Select at Least One Region"
    ValidationGroup="ModalValidation"
    style="display: none"></asp:CustomValidator><ajax:ValidatorCalloutExtender ID="vcRegions" runat="server"
    TargetControlID="cvRegions" 
    PopupPosition="BottomLeft"
    WarningIconImageUrl="~/Images/Icons/warning.png"
    CloseImageUrl="~/Images/Icons/exit.png"></ajax:ValidatorCalloutExtender>

Here is the code for the client side function which performs the actual validation:

function ValidateCheckBoxList(sender, args)
{
    args.IsValid = false;
    var checkedBoxes = $('[id$=Regions] input[type=checkbox]:checked');
    if (checkedBoxes.length > 0) {
        args.IsValid = true;
    }
}

When I click the save button on my page I get the following error:

0x800a139e - JavaScript runtime error: Sys.ArgumentUndefinedException: Value cannot be undefined

Parameter name: id

The problem appears to be that the ValidatorCalloutExtender requires the ControlToValidate property of the CustomValidator to be specified.  When I remove the ValidatorCalloutExtender the actual CustomValidator works as expected.  I made a mock solution to the problem by inserting a TextBox with zero width/no border and attaching the CustomValidator to that control.  While the validator callout shows next to the hidden TextBox it does the job.

Does anyone have a solution to this problem without the crazy hack?