[plt-scheme] segfault using enter!

hi,

mzscheme (both 4.1.5 and 4.1.5.5 built from today's svn, on debian sid)
segfaults systematically under the following scenario:

 ;; test.ss
 (require (planet plai/plai:1:1))

 (define-type FOO
   (foo (n number?)))
 ;; end of test.ss

now in mzscheme:

 > (enter! "test.ss")
 > (foo 3)
 #(struct:foo 3)
 > (enter! #f) ;; optional, the segfault happens without this too
 ;; modify test.ss so that it gets re-loaded
 > (enter! "test.ss")
 [re-loading /home/jao/tmp/test.ss]
 > (foo 3)
 SIGSEGV fault on 0x100004

the sigsev address seems to always be the same, regardless of the exact
definition of FOO or the argument passed to foo in the second
invocation. the modification to test.ss seems irrelevant too (just
touching it so it gets reloaded triggers the problem).

i haven't been able to find a simpler test case triggering the bug, or
to obtain more information on the root cause (i tried to run mzscheme
under gdb, but it segfaults on startup) -- any ideas on how to
investigate the issue welcome!

thanks!
jao

[not sure whether this question belongs to plt-devel; please feel free to
follow-up there]