It looked like the HQ was removed from the map. Hard to be 100% sure with the fog of war.
It happened also with a combat unit and this one was removed from the map though.
I wouldn't regard this as a significant issue at all.
I have seen very small fragments just about cling to the map as strength 1 or even strength 0 units, but all things considered these can be brushed aside at the cost of very few AP, and in any case there'd be prisoners and debris and junk to deal with before the advance can properly get going again.
BTW I'm relieved to say that attacking a strength zero unit does NOT result in a division by zero error!