I tried to check out an archive for a not existing path from the repository and I expect a GitCommandError exception.

I got following TypeError exception:
"Expected bytes or text, but got <cStringIO.StringO object at 0x7fabac6f97a0>"

I did following:
fpt = StringIO()
-- (used as a temporary storage for the tar archive)
self._repository.archive(fpt, treeish=version_identifier, format="tar", path=sub_path)
-- (where sub_path points to an not existing path)

The cause of the problem was, the Constructor of the GitCommandError could not handle the cStringIO object. (a normal file pointer may be faulty too).
I suppose it should set its stdout variable to None in case of not handled types. Or it should be adjusted when raising the exception.