[plt-scheme] Dialog% not accepting frame% subclasses as parent?

I'm confused - an instance of a subclass of frame% doesn't seem to get 
accepted by a dialog% instance as initialization argument.

something like

(instantiate my-dialog% () (parent instance-of-my-frame-subclass) 
(label ""))

yields an error that parent expects an argument of class frame%, 
dialog% or #f.
  (is-a? instance-of-my-frame-subclass frame%) == > #t

Is that behavior intended? How would I pass a custom subclass as parent 
to a dialog?

Another question: Is it possible to display custom sheets on OS X?

Thanks,

Erich