Incomplete rendering of AJAX UpdatePanel using OutputCache

Question

I have a panel with certain properties nested within an UpdatePanel I use for AJAX:

        <asp:UpdatePanel ID="updPanel" runat="server">
            <ContentTemplate>            


<asp:panel ID="Panel2" Width="106" defaultbutton="ibSearchSubmit" runat="server">

Usually, the panel renders correctly as follows:

<div id="ctl00_Panel2" onkeypress="javascript:return WebForm_FireDefaultButton(event, 'ctl00_ibSearchSubmit')" style="width:106px;">

However, I am using OutputCache with the following settings:

<%@ OutputCache Duration="43200" VaryByParam="*" Location="Server" %>

And, sometimes, when a cached version of a page is loaded, the panel is rendered incompletely:

<div id="ctl00_Panel2">

What can I do to get it to render properly without losing the UpdatePanel/AJAX functionality?