traceback when launching "checkbox-cli slave"

There was a new build of checkbox-ng and python3-checkbox-ng this morning. I updated my systems and when I try to launch slave, I get this traceback:

ubuntu@mayapple:~$ checkbox-cli slave
Traceback (most recent call last):
  File "/usr/bin/checkbox-cli", line 11, in <module>
    load_entry_point('checkbox-ng==1.8.0.dev0', 'console_scripts', 'checkbox-cli')()
  File "/usr/lib/python3/dist-packages/checkbox_ng/launcher/checkbox_cli.py", line 123, in main
    subcmd.invoked(ctx)
  File "/usr/lib/python3/dist-packages/checkbox_ng/launcher/slave.py", line 95, in invoked
    strategy = RemoteDebRestartStrategy()
  File "/usr/lib/python3/dist-packages/plainbox/impl/session/restart.py", line 212, in __init__
    self.session_resume_filename = self.get_session_resume_filename()
  File "/usr/lib/python3/dist-packages/plainbox/impl/session/restart.py", line 248, in get_session_resume_filename
    return os.path.join(cache_dir, 'session_resume')
  File "/usr/lib/python3.6/posixpath.py", line 80, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType

ubuntu@mayapple:~$ dpkg -l |grep -e checkbox -e plainbox
ii checkbox-ng 1.7.0+git202004030833+pkg143~ubuntu18.04.1 all CheckBoxNG test runner
ii plainbox-provider-certification-server 0.47.0+git202003210220+pkg134~ubuntu18.04.1 all Server Certification provider for Plainbox
ii plainbox-provider-checkbox 0.51.0+git202004021358+pkg132~ubuntu18.04.1 amd64 CheckBox provider for PlainBox
ii plainbox-provider-resource-generic 0.43.0+git202003051405+pkg87~ubuntu18.04.1 amd64 CheckBox generic resource jobs provider
ii python3-checkbox-ng 1.7.0+git202004030833+pkg143~ubuntu18.04.1 all CheckBoxNG test runner (Python 3 library)
ii python3-checkbox-support 0.44.0+git202003201315+pkg113~ubuntu18.04.1 all collection of Python modules used by PlainBox providers