[racket] strange interference with prompt/abort

Hello,

learning usage of prompt/abort i've met this
strange behavior (code reduced for this example,
thus not really sensy):

(define (·first occurrences x L)
     (define R (filter (λ(ix) (set-member? (list-ref L ix) x)) L))
       (take R (min (length R) occurrences)))
(print (·first 5 (set #\q #\a #\h) "this is a sequence."))

(prompt (+ 2 (abort 5)))


is failing (in DrRacket) with:


call-with-continuation-prompt: contract violation
   expected: continuation-prompt-tag?
   given: #\q
   argument position: 2nd
   other arguments...:
    #<procedure>
    #<procedure:...cket/control.rkt:208:45>

The 'prompt line alone, taken from the
reference, is working perfectly, as in some
other environments also. What is wrong here ?

Regards, Joost Behrends