ValidatorCalloutExtender not working as required

Hi All,

 I am working on ValidatorCalloutExtender.

 I have tried using below code and while throwing error message in callout its just says * not the error message entered in RequiredField Validator.

 The textbox and validaion controls are inside GridVIew Footer template

 See the code below.

<FooterTemplate><asp:TextBox runat="server" ID="txtPONumberF"></asp:TextBox><asp:RequiredFieldValidator OnPreRender="Req_PreRender" ID="Req1" runat="server" Display="None" ControlToValidate="txtPONumberF" ErrorMessage="User Name is required." ToolTip="User Name is required." ValidationGroup="PO">*</asp:RequiredFieldValidator><ajaxToolKit:ValidatorCalloutExtender PopupPosition="BottomLeft" runat="server" TargetControlID="Req1" ID="Validator1"></ajaxToolKit:ValidatorCalloutExtender></FooterTemplate>

 Please advice where it goes wrong

 Thanks in Advance

 PradeepKumar Gnanavel