Can't purchase base game on Steam.

I've been enjoying, via Steam, the free multiplayer mode of Offworld for a few days now and was looking forward to purchasing the base game so I can get some single player in, but the free download is showing in my library as "Offworld Trading Company" and the base game is showing in the store as "Offworld Trading Company" so I'm unable to purchase through the store, as it's claiming it to be something I already own, despite the fact that the in-game menus are telling me that I need to purchase the base game to access single player.

Please advise.