[Zope] What's wrong with this code?

[Luke Tymowski]

Hello,

When:
<dtml-unless renderedheader>
<dtml-call "REQUEST.set('renderedheader',1)">
blah blah
</dtml-unless>

Is put in the standard_html_header object it behaves correctly:
I want my standard header and footer objects to render when pages 
containing them are published.
I don't want my standard header and footer objects to render when

When that same code is put in the standard_html_footer object it never is 
published.

If I preview just the standard_html_header it works.
If I preview just the standard_html_footer it works.

If I create a simple page like:
<dtml-var standard_html_header>
<p>This is some text.</p>
<dtml-var standard_html_footer>

The footer doesn't get published.

I'm sure it's something obvious, but I've spent a few hours going through 
everything I can find and haven't got anywhere.

thanks,

Luke