Reusing unique districts/buildings = red exclamation mark

[chiphead]

I've created a mod (Brazil All Uniques) that gives Brazil the ability to build, among other things, all the unique districts and buildings. Some of them, like the Mbanza and Stave Church, show up in-game as large red exclamation marks. I understand this is because the engine can't load the correct assets, but is there a way to remedy this? I've been looking through the artdefs and not found an obvious link between building and civilization that could be the cause.