Binding definition error with programmed binding classes

[Mike Pokraka]

Hi all, 
640/NW04s/ECC5 SP14 system, I'm trying to use a programmed container->container binding class. 

I've created my class implementing interface IF_SWF_IFS_BIND_TRANSFORM_CONT, but when I try to put it into the binding editor, it complains with:
SWF_BND_001/100: Error when executing binding operaton 'CNT'

I've tried all sorts, even gone as far as implementing the presentation interface and giving it a little icon in the binding editor (nice touch!). 

Looks nice, but still no joy so back to basics: I figured to make sure there's no problem with my code I created two completely empty classes, one for Expression->Expression binding and one for Container->Container binding. The binding editor is happy with the expression binding class, but still gives the same message about the container binding class. 

Any input welcome...
Cheers
Mike 
(Cross-posted to SAP-WUG and SDN-BPM)