All builds fail with Assertion error

I'm honestly not sure what I've done, although I have exported the base directory as an NFS share in anticipation of future system updates. All my builds now fail with the following error:

```
Traceback (most recent call last):
File "/home/paul/cleanroom/clrm", line 13, in
main.run()
File "/home/paul/cleanroom/cleanroom/main.py", line 72, in run
main(*sys.argv)
File "/home/paul/cleanroom/cleanroom/main.py", line 127, in main
with WorkDir(btrfs_helper,
File "/home/paul/cleanroom/cleanroom/workdir.py", line 71, in __init__
if not umount_all(work_directory):
File "/home/paul/cleanroom/cleanroom/helper/mount.py", line 71, in umount_all
umount(mp)
File "/home/paul/cleanroom/cleanroom/helper/mount.py", line 58, in umount
assert len(mount_points(directory)) == 1
AssertionError
```