rsound-fft-draw does not work about rsound HOT 1 OPEN

(define n (network ()
(a <= sine-wave 440)
(b <= sine-wave 1320)
(c <= sine-wave 17600)
(out = (* 0.2 (+ a b c)))))

(signal-play n)
(rsound-fft-draw (signal->rsound 2048 n))

GIVES AN ERROR

build-array: contract violation
expected a vector,
given: 2048
in: the 1st argument of
(->
(vectorof Integer)
(-> (vectorof Index) any/c)
(struct/c
Array
(vectorof Index)
any/c
(box/c (or/c #f #t))
(-> any)
(-> (vectorof Index) any)))
contract from:
(/Applications/Racket v6.0.1.11/share/pkgs/math-lib/math/private/array/array-struct.rkt
typed-module13)
blaming: /rsound/rsound/draw.rkt
at: /math-lib/math/private/array/array-struct.rkt:15.2

from rsound.