How to validate checkboxlist so that all list items cannot be selected?

<asp:checkboxlist id="chkModuleList" runat="server" xmlns:asp="#unknown">
</asp:checkboxlist>

<asp:customvalidator runat="server" id="cvmodulelist" xmlns:asp="#unknown">
  ClientValidationFunction="ValidateModuleList"
  ErrorMessage="All Checkbox checked" ></asp:customvalidator>

// javascript to add to your aspx page
function ValidateModuleList(source, args)
{
  var chkListModules= document.getElementById ('<%= chkModuleList.ClientID %>');
  var chkListinputs = chkListModules.getElementsByTagName("input");
  var CheckedCHK; 
  for (var i=0;i<chklistinputs mode="hold" />  {
    if (chkListinputs [i].checked)
    {
      CheckedCHK++;
    }
  }
  if(CheckedCHK==chkListinputs .length)
   args.IsValid = false;
  else
   args.IsValid = true;
}