[plt-scheme] Binding C-Structs Containing Fixed-Size Arrays with the FFI

I've poked around a bit more, and I hit the same crash even without
doing the make-cstruct-type stuff. I though that the abstraction
provided by types defined with define-cstruct was 'safe' (since it
doesn't even require (unsafe!)), but this crashes:

#lang scheme/base
(require scheme/foreign)

(define-cstruct _bar
  ((x0 _float)
   (x1 _float)
   (x2 _float)
   (x3 _float)))

(define-cstruct _foo
 ((a _bar)
  (b _bar)))

(define s
 (make-foo (make-bar 10. 20. 30. 40.)
           (make-bar 11. 22. 33. 44.)))

(collect-garbage)
(set-bar-x0! (foo-b s) (begin (collect-garbage) 0) 42.)

When I run it in mzscheme v4.1.4  I get:

SIGSEGV fault on 0x1
pid # 9099 run gdb "gdb ./mzscheme3m 9099" or kill process.

Is this a bug?

    Henk