Cannot find button inside an accordion which is inside an update panel

I have a button inside an accordion which in turn is inside an update panel.

I have a file upload control inside this pane.

  <cc1:AccordionPane ID="pane9" runat="server" Visible="false">
                <Header>
                    Material Number Creation</Header>
                <Content>

                              <asp:FileUpload ID="FileUpload1" runat="server" />
                                
                      <asp:Button ID="btnUpload" runat="server" Text="Submit" CssClass="submit" OnClick="btnUpload_Click"   />  

       </Content>
            </cc1:AccordionPane>

        </Panes>
    </cc1:Accordion>

    </ContentTemplate>
            <Triggers>
                <asp:PostBackTrigger ControlID="btnUpload"  />
                        </Triggers>
     
    </asp:UpdatePanel>

I am getting the following error :

A control with ID 'btnUpload' could not be found for the trigger in UpdatePanel 'UpdatePanel1'.