When trying to register a user, this error occurs:

Traceback (most recent call last):
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/steam/flaskbb/flaskbb/auth/views.py", line 163, in register
    send_activation_token.delay(user)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/celery/app/task.py", line 412, in delay
    return self.apply_async(args, kwargs)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/celery/app/task.py", line 535, in apply_async
    **options
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/celery/app/base.py", line 737, in send_task
    amqp.send_task_message(P, name, message, **options)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/celery/app/amqp.py", line 558, in send_task_message
    **properties
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/messaging.py", line 169, in publish
    compression, headers)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/messaging.py", line 252, in _prepare
    body) = dumps(body, serializer=serializer)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/serialization.py", line 221, in dumps
    payload = encoder(data)
  File "/usr/lib/python3.4/contextlib.py", line 77, in __exit__
    self.gen.throw(type, value, traceback)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/serialization.py", line 54, in _reraise_errors
    reraise(wrapper, wrapper(exc), sys.exc_info()[2])
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/vine/five.py", line 175, in reraise
    raise value.with_traceback(tb)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/serialization.py", line 50, in _reraise_errors
    yield
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/serialization.py", line 221, in dumps
    payload = encoder(data)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/utils/json.py", line 72, in dumps
    **dict(default_kwargs, **kwargs))
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/simplejson/__init__.py", line 397, in dumps
    **kw).encode(obj)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/simplejson/encoder.py", line 291, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/simplejson/encoder.py", line 373, in iterencode
    return _iterencode(o, 0)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/kombu/utils/json.py", line 62, in default
    return super(JSONEncoder, self).default(o)
  File "/home/steam/flaskbb/.venv/lib/python3.4/site-packages/simplejson/encoder.py", line 268, in default
    raise TypeError(repr(o) + " is not JSON serializable")
kombu.exceptions.EncodeError: <User testuser> is not JSON serializable