Why does ASP.NET think my control doesn't exist? - asp.net

Question

I have an ASP.NET form with an UpdatePanel. I have two AsyncPostBackTriggers defined for that UpdatePanel; both are buttons, which both do the same thing (one is at the top of the form, the other is at the bottom of the form), the trigger event name is "Click" for both.
As soon as I load the page, I get the error "A control with ID 'btnSaveChangesBtm' could not be found for the trigger in UpdatePanel 'updIncidentDetails'." This error makes no sense, because I look and there's the control 'btnSaveChangesBtm', clear as day.
Here are the relevant snippets of the markup, in case someone sees something I'm missing. I'm at a loss on this one.
<asp:UpdatePanel ID="updIncidentDetails" runat="server">
<ContentTemplate>
<div id="pnlIncidentDetails" runat="server" class="formPanel">
<h2>Incident Details</h2>
<div id="saveButton" style="float: right; margin-right: 100px; display: none;">
<asp:Button ID="btnSaveChanges" runat="server" OnClientClick="return valEditedFields();" OnClick="btnSubmit_Click" Text="Save Changes" />
<asp:Button ID="btnCancel" runat="server" OnClientClick="return hideEditFields(true);" OnClick="btnCancel_Click" Text="Cancel Changes" />
</div>
...
<div id="saveButtonBtm" style="float: right; display: none;">
<br />
<asp:Button ID="btnSaveChangesBtm" runat="server" OnClientClick="return valEditedFields();" OnClick="btnSubmit_Click" Text="Save Changes" />
<asp:Button ID="btnCancelBtm" runat="server" OnClientClick="return hideEditFields(true);" OnClick="btnCancel_Click" Text="Cancel Changes" />
<br />
</div>
</div>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnSaveChanges" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="btnSaveChangesBtm" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
I first thought it was because I have "display: none;" set on the "saveButtonBtm" <div>, but then I realized the same thing is set on the one at the top ("saveButton"), so I don't think that's it. Any other suggestions?
Here is an image of the error:

Answer 1

The problem was because of the buttons being inside of an AccordionPanel -- I guess that made it so the UpdatePanel couldn't find them. I moved them to outside the Panel and it all works.