ModalPopupExtender OK Button click event not firing?

Aspx

<ajax:ModalPopupExtender runat="server" ID="modalPop" 
            PopupControlID="pnlpopup" 
            TargetControlID="btnGo"
              BackgroundCssClass="modalBackground"
             DropShadow="true"
             CancelControlID="btnCancel" X="470" Y="300"   />


//Codebehind    
protected void OkButton_Clicked(object sender, EventArgs e)
    {

        modalPop.Hide();
        //Do something in codebehind
    }

And don’t set the OK button as OkControlID.