problem with poseAs

[Lloyd Dupont]

I'm trying to port an application write on MacoSX to GNUstep/Windows.
The application uses many MacOSX 10.3 features (I mean method and classes 
added in 10.3).

I had a runtime error because of such a missing accessor for a new 
defaultTabInterval property in NSMutableParagraphStyle.

So I decided to simply write a superclass and use the 'poseAs ' mechanism.
However, when I did so, I had this puzzling error message issued by the 
runtime (and followed by an exit(3)):
Assertion failed: impostor->instance_size == super_class->instance_size, 
file class.c, line 638

I'm kind of puzzled, couldn't you have new instance variable in a posing 
subclass?
That's the whole advantage compare to categories...

How could I work around that?
I need paragraph style information extensoively and absolutely, I can't 
simply remove the code using it....