I have to admit, it's been a while since I had the player ship in such a state that the shields were destroyed.
I'm afraid this is, sort of, a known problem. It's the reason why you don't (anymore0 see ships with shields with a maximum strength of 0.
I'll guard against it in a future DiamondBC release.
But for the moment all I can say is, try not to get your shield generators destoryed. ;)