[Zope] <dtml-var x missing> doesn't work?

Hi, I'm doing a back link to go on a page (yes, I know browsers have back buttons, but some people don't like to use them ;-) which is easy enough: <A href="<dtml-var "REQUEST.environ['HTTP_REFERER']">">Back</A> except that if there is no referer, you get a key error. So I thought I'd try the following: <A href="<dtml-var "REQUEST.environ['HTTP_REFERER']" missing='/'>">Back</A> But I still get a key error. I thought the 'missing' attribute was supposed to catch that? Chris