Racket code runs but sometimes does not produce output.

[fuxoft]

I ran this exactly same code several times (preceded by ./run racket):

#lang racket
(struct item (weight value) #:transparent)
(define (maximum-value maximum-weight items)
  (for/fold ([maxval 0] #:result maxval) ([combo (in-combinations items)])
    (values (if (<= (apply + (map item-weight combo)) maximum-weight)
      (let ([curval (apply + (map item-value combo))])
        (if (> curval maxval) curval maxval)) maxval))))
(display (format "max value: ~s" (maximum-value 10 (list (item 5 10) (item 4 40) (item 6 30) (item 4 50)))))

At first it produced the correct output (max value: 90) but afterwards it repeatedly said "Your racket(8.3.0) code ran without output".

Again, it was the EXACT same code.

Now, even even the following trivial code:

#lang racket
(display "hello")

...produces no output, only the messge "Your racket(8.3.0) code ran without output".