Custom battle arbitrarily changes players sides at start up.

[ksbeargame]

I accepted a challenge in which I was supposed to be the Rus side in a game with Rus vs Vikings. I accepted the challenge and was presented with the RUS "buy" screen in which I was able to buy Rus units. However, for some reason, when my opponent returned to the game after setup, I am now the Viking side. How does that happen? This is the Steam versionm and I am up to date.