Traceback (most recent call last):
  File "/snap/hostname-desktop-installer/511/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquitycore/async_helpers.py", line 26, in _done
    fut.result()
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquity/server/controllers/mirror.py", line 141, in _apply_apt_config
    await self.apt_configurer.apply_apt_config(self.context)
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquity/server/apt.py", line 190, in apply_apt_config
    self.configured_tree = await self.setup_overlay([self.source])
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquity/server/apt.py", line 167, in setup_overlay
    lowerdir = lowerdir_for(lowers)
  File "/snap/hostname-desktop-installer/511/usr/lib/python3.8/functools.py", line 875, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquity/server/apt.py", line 92, in _lowerdir_for_lst
    return ':'.join(reversed([lowerdir_for(item) for item in lst]))
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquity/server/apt.py", line 92, in <listcomp>
    return ':'.join(reversed([lowerdir_for(item) for item in lst]))
  File "/snap/hostname-desktop-installer/511/usr/lib/python3.8/functools.py", line 875, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/snap/hostname-desktop-installer/511/bin/subiquity/subiquity/server/apt.py", line 70, in lowerdir_for
    raise NotImplementedError(x)
NotImplementedError: None