JavaScript for Clientside validation for asp.net Single-CheckBox using CustomValidator

<script  language="javascript" type ="text/javascript" >
       function checkAgreement(source, args)
        {
         var elem = document.getElementById("cbxPDCheque");
         if (elem.checked)
          {
          args.IsValid = true;
          }
          else
          {
          args.IsValid = false;
          }
        }
        </script>

<asp:CheckBox ID="cbxPDCheque" runat="server" AutoPostBack="True" Text="PD Cheques" />
  <asp:CustomValidator ID="CustomValidator1" runat="server" EnableClientScript ="true"
  ErrorMessage="Select PD Cheques."  ClientValidationFunction ="checkAgreement" >*</asp:CustomValidator>
  <asp:ValidationSummary ID="ValidationSummary1" runat="server" ShowMessageBox="True" ShowSummary="False" />
  <asp:Button ID="Button1" runat="server" Text="Button" />

http://www.codeproject.com/Questions/188889/JavaScript-for-Clientside-validation-for-asp-net-S