[racket] strange interference with prompt, correcture

Hello,

reducing the original code to the example
for containing the problem, i've committed a
slip. The example code should be:

(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) (string->list "this is a sequence.")))

(prompt (+ 2 (abort 5)))

(with 'string->list added). It
is failing (in DrRacket) the same way:


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