Importing old archives fails

Running ./manage.py hyperkitty_import -l mailman-users@mailman3.org <Downloaded mbox> fails:

Importing from mbox file /home/sim/Downloads/mailman-users@mailman3.org.mbox to mailman-users@mailman3.org
|Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/sim/dev/mailman/hyperkitty/example_project/env/lib/python3.6/site-packages/Django-1.11.11-py3.6.egg/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/home/sim/dev/mailman/hyperkitty/example_project/env/lib/python3.6/site-packages/Django-1.11.11-py3.6.egg/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/sim/dev/mailman/hyperkitty/example_project/env/lib/python3.6/site-packages/Django-1.11.11-py3.6.egg/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/sim/dev/mailman/hyperkitty/example_project/env/lib/python3.6/site-packages/Django-1.11.11-py3.6.egg/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/home/sim/dev/mailman/hyperkitty/hyperkitty/management/commands/hyperkitty_import.py", line 281, in handle
    importer.from_mbox(mbfile)
  File "/home/sim/dev/mailman/hyperkitty/hyperkitty/management/commands/hyperkitty_import.py", line 154, in from_mbox
    add_to_list(self.list_address, message)
  File "/home/sim/dev/mailman/hyperkitty/hyperkitty/lib/incoming.py", line 158, in add_to_list
    encoding=encoding, content=content.encode('utf-8'))

I'm using the master branches of mailmanclient, hyperkitty and django-mailman3