It's apparantly a missing texture for one of the units. Aymar has probably fixed this already. But he might not.

I guess it's one of the Karthadashtim units you haven't fought before or a mercenary unit you haven't used yet. I any case it will be fixed in the next patch. However the only way to avoid this for now is by autocalc.

Anyway thanks for reporting, if you see another bug keep reporting even tough you think it might have been reported already.