OSPF NSSA default routes

[HugoFarquharSelfe]

I am configuring Fortigates as OSPF NSSA ABRs.  I would like to advertise a default route into the NSSA as long as the ABR has received a default route in the backbone area.

With standard areas, this is easily managed by simply adding 

config router ospf  set default-information-originate enable

end

However, I am unable to find a way to get a default route advertised into an NSSA except by using "set stub-type no-summary", which is not what I want at all.  As far as I can see, that generates a default route on the ABR rather than advertises one that the ABR has learnt from elsewhere, and it doesn't stop generating it when the firewall has not learnt the route from upstream, which is what I need.

I have tried every combination I can think of for the various "set nssa-..." commands but nothing seems to work.

This is the same on various 5.2 versions, physical and virtual.

Any clues, anyone?