Yeah, I'm getting the same thing. Steam has some jacked-up issues with some DLC and the base not being properly associated. I'm betting that it has something to do with the fact that the base is in the "profile features limited" state, but of course it could be something else entirely. It may be the fact that the base is free...I had this happen to me with Frontline Tactics. Still have two of the "complete pack" DLCs for that game that are impossible to give away on SG.

If you still want to GA it, you could always give it away on Indiegala. Or, just set up a trade for it and exchange keys directly. If you decide you want to trade for it, let me know - I might be interested.