CustomValidator for Radio Button

I have a CustomValidator for a Radio Button that works fine with IE, but
doesn't trigger with Firefox or Netscape.

I have been stuck on this one for 3 weeks; any suggestions greatly
appreciated.

Thanks!

<script language="javascript">
function ValidateRadiobutton(oSrc, args) {
if(document.getElementById("<%=rbChoiceA.ClientID%>").checked == false
&&
document.getElementById("<%=rbChoiceB.ClientID%>").checked ==
false){
args.IsValid = false;
}
}
</script>

<asp:CustomValidator id="vld1PaymentType"
ClientValidationFunction="ValidateRadiobutton" runat="server"
ErrorMessage="Payment Type must be selected"
CssClass="ValidationMessage"></asp:CustomValidator>


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