Human-only Unit CTD

I've been playing the "Fall of Rome" Mod, and have encountered an odd CTD. In essence the problem is this: For certain units (and very common ones at that - such as ballista, peasant, etc), if they are present in my army, the game CTDs whenever I try to use them on the battle map - although there's no crash if I "auto-resolve". However, if the AI owns them, the game works fine. I can see the enemy units on the battle map, and they look normal, so it wouldn't seem to be a graphics problem.

Does anyone have any idea what is causing this "human-only" bug? (and what I should do to fix it).

Edit: Now I'm totally confused. I went back to an earlier game which I played as the Danes, and they HAVE a Ballista and can USE it on the battle map!! How could a unit cause a CTD when used by one human faction (the CTD happens when I play as the Romans) but work for another??