ImportError: No module named message

When I install hyperkitty from pip and then try to do "migrate", I get this error:

Step 32/39 : RUN su -s /usr/local/bin/python uwsgi /usr/local/mailman-django/manage.py migrate
 ---> Running in 6e485fb4c1eb
Traceback (most recent call last):
  File "/usr/local/mailman-django/manage.py", line 10, in 
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 338, in execute
    django.setup()
  File "/usr/local/lib/python2.7/site-packages/django/__init__.py", line 27, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/usr/local/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models()
  File "/usr/local/lib/python2.7/site-packages/django/apps/config.py", line 202, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/local/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/local/lib/python2.7/site-packages/hyperkitty/models/__init__.py", line 26, in 
    from .email import Email, Attachment
  File "/usr/local/lib/python2.7/site-packages/hyperkitty/models/email.py", line 25, in 
    from email.message import EmailMessage
ImportError: No module named message
The command '/bin/sh -c su -s /usr/local/bin/python uwsgi /usr/local/mailman-django/manage.py migrate' returned a non-zero code: 1

This is part of my Docker build. Dockerfile attached