[plt-scheme] Object Inheritance

Hello,

I try to extend a basic object. Here, an example :

> (define pg-frame%
>   (class frame%
>     (new message% (parent this) (label "Message"))
>     (super-new)))

> (define f (new pg-frame% (label "Frame")))
> (send f show #t)

But i have this error :
send: target is not an object: #<undefined> for method: get-container

The message% parent has to be an instance from frame%. So "this" is
not correct. But it is possible to do what i want ?

Thanks,
Laurent