Sort contacts by "suffix" and secondarily by "full name"?

I have divided my contacts into several groups — I realise that “category” serves this purpose, but I had already adapted “suffix” (which is otherwise unused) to do it.

Now I would like the contacts to be ordered by “full name” within each suffix value, but this doesn’t happen. When I sort by suffix, the contacts within each suffix value seem to be ordered by email-address.

Is there a way to achieve this? Using version 7.2.