control inherited from Panel doesn't generate end tag

[Daniel vom Saal]

I have a control that is inherited from Panel that is very, very simple: it
merely adds some attributes in the New() function. However, it doesn't seem
to be generating the end tag (</div>) where expected. Basically the page
contains

<cc1:CustomPanel id=cp1 runat=server>
< other tags >
</other tags >
</cc1:CustomPanel>

and I expected

</div id=cp1 style="the custom attributes">
<other tags>
</other tags>
</div>

but I never get the closing </div> tag (the opening tag generated
perfectly).

Am I missing something simple?