Tangent space computation needs an UVMap error

This has me stumped. I’m trying to export a skeletal mesh as an FBX file —

RuntimeError: Error: Tangent space computation needs an UVMap, “(null)” not found, aborting

It looks self explanatory. However, the skeletal mesh does have a UV map. As a test, I deleted the mesh and replaced it with a simple cube which I then UV unwrapped. The error still appears. Deleting all the faces so that there’s no mesh at all still produces the error. What’s going on here?