[plt-scheme] I can't seem to match structures in the html library

I tried to post this via gmane but it never seemed to get here, trying
again...

  Welcome to MzScheme version 204, Copyright (c) 1995-2003 PLT
  > (require (lib "match.ss"))
  > (require (lib "html.ss" "html"))
  > (define (f x) (html-element? x))
  > (f 3)
  #f
  > (define g (match-lambda [($ html-element attrs) #f]))
  STDIN::151: match: not a defined structure in: html-element

Why can't I match on the structure html-element?  Am I doing something
stupid?  I've used the pattern-matching libraries on my own structures
without any problems.

Thanks,
Dan

-- 
http://www.dfan.org