I've had the null thing consistently with FB. silver, gold or plat does not make a difference. Never or hardly ever happened with FD.

Thinking of it: a while ago I got an email with a new pin (although i didn't ask for one).
The old one was still working (as they wrote) so I did not bother.
Perhaps this null null null thingy is "the system" punishing me for not using the new pin ....