[Zope] What's up with the format commands?

I'm trying to use fmt=url_quote to munge the value of a property into something that I can put into an url (obviously :-), yet zope is giving me grief: This... <dtml-var standard_html_header> <dtml-let foo="absolute_url"> <dtml-var foo fmt=url_quote> </dtml-let> <dtml-var standard_html_footer> Gives me this error... Error Type: TypeError Error Value: not all arguments converted [snip] File /usr/lib/zope/lib/python/DocumentTemplate/DT_Let.py, line 145, in render (Object: foo="absolute_url") File /usr/lib/zope/lib/python/DocumentTemplate/DT_Var.py, line 313, in render (Object: foo) (Info: ('foo', 'http://antares/pcgi/kaos/memepool', {'fmt': 'url_quote', '': 'foo'})) TypeError: (see above) John.