[racket] Using big-bang with picts

Hi All,

Is there a way to use big-bang with picts?
In other words, how can I convert a pict into something that big-bang
will display?

The attempt below fails with this error:

    collects/racket/private/class-internal.rkt:4387:0: send: no such method
    method name: copy
    class name: bitmap%

#lang racket
(require pict (only-in 2htdp/universe big-bang on-tick to-draw))

(define (world->image w)
  (pict->bitmap (circle w)))

(big-bang 100
          [on-tick add1]
          [to-draw world->image])


--
Jens Axel Søgaard