Well anyway what you can do is reinstall the game to make sure you get the updated DLC files or manually go in and delete the faulty DLC zip and have Steam download it again, thought however it would be Charlemagne with its units but apparently not so I can't tell you exactly which one it is, maybe Iberian Unit pack?
Ninja'd: Fixed it for one, so I am pretty sure this is the case, you can either delete the DLC files and have Steam download it again or to be extra sure, just ask steam to delete the game and then redownload it.