Technical issues with Heroes III.

Why did the number of creatures become negative? I'm playing against the computer, and I spent many nights leveling up and building an army. I gathered around 36-37k skeletons. But on the way to another hero, this number became -39766. It's frustrating, and I don't have an autosave with a normal number of creatures. Why does this happen, and how can I avoid it? It seems like there's no limit to the number of creatures (I created a hero with 7 slots, each with 9999, and combined them). Please help.