a.rb

root@Debian-70-wheezy-64-minimal:~# jupyterhub-singleuser 
Traceback (most recent call last):
  File "/home/epinux/dev/ipython/src/traitlets/traitlets/traitlets.py", line 432, in __get__
    value = obj._trait_values[self.name]
KeyError: None

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/epilib/Envs/env1/bin/jupyterhub-singleuser", line 4, in <module>
    main()
  File "/home/epilib/Envs/env1/lib/python3.4/site-packages/jupyterhub/singleuser.py", line 221, in main
    return SingleUserNotebookApp.launch_instance()
  File "/home/epilib/Envs/env1/lib/python3.4/site-packages/IPython/config/application.py", line 573, in launch_instance
    app.initialize(argv)
  File "<string>", line 2, in initialize
  File "/home/epilib/Envs/env1/lib/python3.4/site-packages/IPython/config/application.py", line 75, in catch_config_error
    return method(app, *args, **kwargs)
  File "/home/epilib/Envs/env1/lib/python3.4/site-packages/IPython/html/notebookapp.py", line 1015, in initialize
    self.init_webapp()
  File "/home/epilib/Envs/env1/lib/python3.4/site-packages/jupyterhub/singleuser.py", line 191, in init_webapp
    s['user'] = self.user
  File "/home/epinux/dev/ipython/src/traitlets/traitlets/traitlets.py", line 438, in __get__
    % (self.name, obj))
traitlets.traitlets.TraitError: No default value found for None trait of <jupyterhub.singleuser.SingleUserNotebookApp object at 0x7fdc5ae37fd0>

If you suspect this is an IPython bug, please report it at:
    https://github.com/ipython/ipython/issues
or send an email to the mailing list at ipython-dev@scipy.org

You can print a more detailed traceback right now with "%tb", or use "%debug"
to interactively debug it.

Extra-detailed tracebacks for bug-reporting purposes can be enabled via:
    c.Application.verbose_crash=True

root@Debian-70-wheezy-64-minimal:~#