Ajax ModalPopup error?

ModalPopUp coding

<asp:RadioButton ID="modalPanel" runat="server" Text="Show Modal"/><asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="popUp"
                                     TargetControlID="modalPanel" CancelControlID="modalClose" OkControlID="Button2"  BackgroundCssClass="modalBackground" DynamicServicePath="" Enabled="True"></asp:ModalPopupExtender>

Ok for button2 clicked

protected void Button2_Click(object sender, EventArgs e)
        {
           custName.Text = "Test modal";

        }

Control Property for button2

<asp:Button ID="Button2" runat="server" Text="Ok" />

apparently the custName textbox did not have the text: "Test modal" after clicking the button 2