Unhandled exception in utils._force_rmtree()

This happened today during a GNOME build:

[--:--:--] BUG     [apps/qqwing.bst]: Build

    An unhandled exception occured:
    
    Traceback (most recent call last):
      File "/codethink/GNOME/buildstream/buildstream/_scheduler/job.py", line 265, in child_action
        result = self.action(element)
      File "/codethink/GNOME/buildstream/buildstream/_scheduler/buildqueue.py", line 34, in process
        element._assemble()
      File "/codethink/GNOME/buildstream/buildstream/element.py", line 1135, in _assemble
        cleanup_rootdir()
      File "/codethink/GNOME/buildstream/buildstream/element.py", line 1058, in cleanup_rootdir
        utils._force_rmtree(rootdir)
      File "/codethink/GNOME/buildstream/buildstream/utils.py", line 481, in _force_rmtree
        shutil.rmtree(rootpath, **kwargs)
      File "/usr/lib/python3.5/shutil.py", line 480, in rmtree
        _rmtree_safe_fd(fd, path, onerror)
      File "/usr/lib/python3.5/shutil.py", line 418, in _rmtree_safe_fd
        _rmtree_safe_fd(dirfd, fullname, onerror)
      File "/usr/lib/python3.5/shutil.py", line 418, in _rmtree_safe_fd
        _rmtree_safe_fd(dirfd, fullname, onerror)
      File "/usr/lib/python3.5/shutil.py", line 438, in _rmtree_safe_fd
        onerror(os.unlink, fullname, sys.exc_info())
      File "/usr/lib/python3.5/shutil.py", line 436, in _rmtree_safe_fd
        os.unlink(name, dir_fd=topfd)
    IsADirectoryError: [Errno 21] Is a directory: 'mount'