okcontrolid's onclick event not firing in ModalPopupExtender

Question

Hi,

    I have problem in ModalPopupExtender. Here is my code-

   <ajaxtoolkit:modalpopupextender id="ModalPopupExtender" runat="server" 

            cancelcontrolid="LinkButton1" dropshadow="false" okcontrolid="btnSubmit"                 X="200" Y="100" popupcontrolid="Panel1" targetcontrolid="imgSchedul">

<asp:Button ID="btnSubmit"  runat="server" Text="Submit" 

                           onclick="SubmitData" ValidationGroup="MainValidation"/>

protected void SubmitData(object sender, EventArgs e)

{

//code for submit data.

}

  I have written onclick event for btnSubmit in code behind file. Now my problem is, this event is not firing when I click Submit Button on Panel.

Thanks,

 VIkram