Ajax Auto Extender and Custom Validator

I am using asp.net Grid view which has Text box mapped to autoextender. I am unable to trigger custom validator. Though script is called , it doesnot display the error messgae. Following is the code used.

<script type="text/javascript">
var isItemSelected = false;
function onItemSelected() {
isItemSelected = true;
}

function checkItemSelected(sender,args) {
alert(sender.id);


if (!isItemSelected) {
// alert('inside if');
// txtInput.focus();

args.isValid = false;
alert(args.Value + isItemSelected);
return args.isValid;
}
else {
//reset isItemSelected
isItemSelected = false;
}
}

</script>

<asp:TemplateField >
<ItemTemplate>
<asp:TextBox ID="txtEmp" runat="server" ></asp:TextBox>

<asp:AutoCompleteExtender ServiceMethod="GetEmpList"
MinimumPrefixLength="1"
CompletionInterval="10"
EnableCaching="false"
CompletionSetCount="1"
TargetControlID="txtEmp"
ID="AutoCompleteExtender4"
runat="server"
OnClientItemSelected="onItemSelected"
FirstRowSelected="false"
>
</asp:AutoCompleteExtender>

<asp:CustomValidator id="DisValidator"
runat="server" ControlToValidate="txtEmp" ValidateEmptyText="false" ClientValidationFunction="checkItemSelected"
Display="Dynamic" ErrorMessage="Please select item" ForeColor ="Red" Enabled="true">*</asp:CustomValidator>
</ItemTemplate>