convertFoundSet(forms.foo.foundset, foo_to_bar) gives zero-s

Subject says it all, well mostly.

If I try convertFoundSet(forms.foo.foundset, foo_to_bar), where there are known to be child records, I always get an empty answer.

Is this type of construct (explicit reference to a form, rather than referencing the current “foundset”) supposed to work?

Thanks,
Neale.