Passing an object with an invalid type, like None, to os.path.join after the first argument results in the following exception: 

TypeError: join() argument must be str or bytes, not 'NoneType'

Exception message can be updated to show that os.PathLike objects can be used:

TypeError: join() argument must be str, bytes or os.PathLike object, not 'NoneType'

This is also more consistent with the TypeError raised by os.fspath.