Failure to serialize lists about mongoengine-goodjson HOT 3 CLOSED

from mongoengine import *
import mongoengine_goodjson as gj
from datetime import datetime

connect(host='mongodb://<ip>:27017/test')

STATES = ('PENDING','SOMETHING')
class Scanner(gj.Document):
    session_name = StringField(required=True, unique=True)
    cid = StringField()
    zzz = StringField()
    xxx = StringField()
    yyy = StringField()
    state = StringField(choices=STATES, default=u'PENDING')
    created_at = DateTimeField(default=datetime.now)
    online_at = DateTimeField()
    started_at = DateTimeField()
    stopped_at = DateTimeField()
    scan_start = DateTimeField()
    scan_complete = DateTimeField()
    destroyed_at = DateTimeField()

s1 = Scanner(session_name='s1', online_at=datetime.now())
s1.save()

s2 = Scanner(session_name='s2', started_at=datetime.now())
s2.save()

>>> Scanner.objects[0].to_json()
'{"created_at": "2017-04-03T09:45:55.926000", "session_name": "s1", "online_at": "2017-04-03T09:45:55.926000", "state": "PENDING", "id": "58e27c4407d32b9fd13f98b0"}'
>>> Scanner.objects[1].to_json()
'{"created_at": "2017-04-03T09:46:00.526000", "session_name": "s2", "started_at": "2017-04-03T09:46:00.526000", "state": "PENDING", "id": "58e27c4807d32b9fd13f98b1"}'
>>> Scanner.objects[0:1].to_json()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/<user>/.pyenv/versions/automation/lib/python2.7/site-packages/mongoengine_goodjson/queryset.py", line 80, in to_json
    lst = self.as_pymongo()
  File "/Users/<user>/.pyenv/versions/automation/lib/python2.7/site-packages/mongoengine_goodjson/queryset.py", line 70, in as_pymongo
    item[name] = self.__get_doc(fld, item[name])
KeyError: 'zzz'