Getting error when use AJAX UpdatePanel

Hello,
  On a Button_Click event  I need to make visible Panel control that contains User control. Both Button and
Panel controls wrapped by UpdatePanels. I defined a Trigger on the UpdatePanel that contains Panel to be  visible in response to the Click event. but when I click on the button I get the error msg.

Here the code:

For the Button

<ajax:UpdatePanel ID="UpdatePanel1" runat="server">
                                <ContentTemplate>
                                    <asp:Button ID="btnAddNewRecord" runat="server" Text="Add Record" OnClick="btnAddNewRecord_Click" />
                                </ContentTemplate>
                            </ajax:UpdatePanel>

And for the Panel that should become visible in response to the Button_Click event:

 <ajax:UpdatePanel ID="upnlAddRecordBase" runat="server">
        <ContentTemplate>
            <asp:Panel ID="pnlAddRecord" runat="server" Height="74px" Visible="False" Width="702px">
                <uc1:AddRecord ID="AddRecord1" runat="server" />
            </asp:Panel>
        </ContentTemplate>
        <Triggers>
          <ajax:AsyncPostBackTrigger ControlID ="btnAddNewRecord" EventName ="Click" />
        </Triggers>
    </ajax:UpdatePanel>

Please help!