It was using the stock Amonkrie files. This one happened on turn 48, the last on happened about mid-way through the last game. I took over and surrendered him to a neutral (just to make things a little more interesting) and haven't had a problem since, but I'd still really like to know what's causing this and how to fix it (if possible).
Very odd! After some sleep I did some more testing and the problem seems to be with one specific file (at least in this case), Vehiclesize.txt which is odd because the last change I made to it was to make the WarStation slightly easier to hit. Other than that I haven't touched that file for quite some time. However when I went back to the version before that change, the div/0 error didn't rear it's ugly head.