[plt-scheme] bitmap% get-loaded-mask not available?

Hi,

I'm loading a transparent gif like this:

(set! pic (make-object bitmap% file-path 'gif/mask))

but in a canvas callback

  (define (paint canvas dc)
         (send dc draw-bitmap pic 0.0 0.0 'opaque (send dc 
get-background) (send pic get-loaded-mask))
         )

the big OS X  plt 202 Scheme pretends

send: no such method: get-loaded-mask for class: bitmap%

even though the manual says this method exists. (I've made sure that 
the GIF is transparent.)

What's wrong?

Regards,

Erich