A control with ID could not be found for the trigger in UpdatePanel

I have an update panel that has UpdateMode of Conditional and ChildrenAsTriggers set to false. I only want a few controls to cause an asynchronous postback:

<asp:UpdatePanel ID="updPnlMain" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
<ContentTemplate>

      // ...
      <asp:Repeater ID="rptListData" runat="server">
          <ItemTemplate>
              <asp:Button ID="btnAddSomething" runat="server" OnClick="btnAddSomething_Click" />
          </ItemTemplate>
      </asp:Repeater>
      // ...
</ContentTemplate>
<Triggers>
    <asp:AsyncPostBackTrigger ControlID="btnAddSomething" EventName="Click" />
</Triggers>
</asp:UpdatePanel>

I am getting the following error when I try and load this page:

A control with ID 'btnAddSomething' could not be found for the trigger in UpdatePanel 'updPnlMain'.

Since my btnAddSomething control is in a repeater and might not be there right away it acts like it is nonexistent. How can I get around this?