Bug in interaction of closures, set! and `fxarithmetic-shift-right'

The following program produces a bogus result:

;; ----------------------------------
(import (rnrs))

(define-syntax trace
  (syntax-rules ()
    ((_ . args)
     (begin
       (for-each display (list . args))
       (newline)))
    #;
    ((_ . args) (begin))))

(define (make-bit-reader port)
  (let ((buf 61) (buflen 8))
    (lambda (count)
      (let ((v (fxbit-field buf 0 count)))
        (trace "BEFORE READ (c=" count "): buf=" buf ", buflen=" buflen)
        (set! buf (fxarithmetic-shift-right buf count))
        (set! buflen (fx- buflen count))
        (trace "AFTER READ: buf=" buf ", buflen=" buflen)
        v))))

(let ((get-bits (make-bit-reader
                 (open-bytevector-input-port #vu8(#b00111101 #b11000011 #b10100000)))))
  (get-bits 3))
;;--------------------------------

Running this with "ikarus --r6rs-script ikarus-set-bug.scm" results in
the following output:

BEFORE READ (c=3): buf=61, buflen=8
AFTER READ: buf=61, buflen=5

Note that the value of `buf' is unchanged after the `set!', even
though the shift amount `count' is 3. When one changes
`fxarithmetic-shift-right' to `bitwise-arithmetic-shift-right', the
problem goes away and the correct output is displayed:

BEFORE READ (c=3): buf=61, buflen=8
AFTER READ: buf=7, buflen=5

This is all with: Ikarus Scheme version 0.0.4-rc1+, 64-bit (revision
1856, build 2009-09-29).

I suspect this is some kind of compiler bug.