Historical Battle Error

After playing the Battle on the river Areios, then quitting the game, I got this error:

Generic Error - Error in Battle Script command 'define_unit_group': unit 'prodromi' is already in a unit group.

This did not cause a CTD, showed up after I quit, as I have the -show err command enabled.

Yes, it's been reported by someone else as well. I presume someone involved with the historical battles will correct it, but it's a very minor bug since it does not affect gameplay.