Unexpected None causes 500

[al4]

Should be 400

[2017-03-29 14:39:14 +0000] [15076] [ERROR] Error handling request /releases
Traceback (most recent call last):
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 135, in handle
    self.handle_request(listener, req, client, addr)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 176, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 2000, in __call__
    return self.wsgi_app(environ, start_response)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1991, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1567, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/orlo/user_auth.py", line 42, in wrapped
    return func(*args, **kwargs)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/orlo/route_releases.py", line 42, in post_releases
    release = create_release(request)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/orlo/util.py", line 41, in create_release
    references = list_to_string(references)
  File "/opt/venvs/orlo/local/lib/python2.7/site-packages/orlo/util.py", line 150, in list_to_string
    return '["' + '", "'.join(array) + '"]'
TypeError: sequence item 0: expected string or Unicode, NoneType found