Validating AJAX Cascading Drop Down Lists?

I am using the AJAX Cascading drop down list but want to add event validation e.g. the compare validators.

As the cascading drop down list requires the page event validation to be disabled what is the best way to do the validation?

Validation Attempt: I have tried to use a custom validator which calls a Javascript function but it doesnt seem to be picking up the control. I get the following error Microsoft JScript runtime error: Object required

function ValidateCostCentCat(source, arguments)
{
var countryList = document.getElementById("ddlCategory");
if (null != countryList)
{
var iValue = countryList.options[countryList.selectedIndex].value;
if (iValue == "Select Category")
{
arguments.IsValid = true;
}
else
{
arguments.IsValid = false;
}
}
}

The mark-up for the custom validator is

<asp:CustomValidator ID="valcustCategory" runat="server" CssClass="error" Display="Dynamic" ValidationGroup="DirectHire" ClientValidationFunction="ValidateCostCentCat"
ErrorMessage="Please select a Cost Centre Category from the drop down list provided.">!</asp:CustomValidator>