mongoengine: Attribute error when using db_field and setting a property

dbfield_bug.py

# Blank to start

from mongoengine import connect, Document, StringField
connect('gist-2944918')


class Kard(Document):
    """
    Represents a card on a Kanban board.
    """
    key = StringField(required=True, unique=True)
    title = StringField()
    _service_class = StringField(required=False, db_field="service_class")

    @property
    def service_class(self):
        return self._service_class or "Default"

Kard.objects.all().delete()

k = Kard(key="IPHONE-1", title="Build an iPhone", _service_class="Epic")
k.save()

k2 = Kard(key="IPHONE-2", title="Build an iPhone 2")
k2.save()

no_class = Kard.objects.get(key="IPHONE-2")  # Works


epic = Kard.objects.get(key="IPHONE-1")  # Attribute error

"""
Traceback (most recent call last):
  File "dbfield_bug.py", line 28, in <module>
    epic = Kard.objects.get(key="IPHONE-1")
  File "/Users/chris/Dropbox/repos/gist-2944918/.fe/lib/python2.7/site-packages/mongoengine/queryset.py", line 777, in get
    result1 = self.next()
  File "/Users/chris/Dropbox/repos/gist-2944918/.fe/lib/python2.7/site-packages/mongoengine/queryset.py", line 957, in next
    return self._document._from_son(self._cursor.next())
  File "/Users/chris/Dropbox/repos/gist-2944918/.fe/lib/python2.7/site-packages/mongoengine/base.py", line 980, in _from_son
    obj = cls(**data)
  File "/Users/chris/Dropbox/repos/gist-2944918/.fe/lib/python2.7/site-packages/mongoengine/base.py", line 801, in __init__
    setattr(self, key, value)
  File "/Users/chris/Dropbox/repos/gist-2944918/.fe/lib/python2.7/site-packages/mongoengine/base.py", line 846, in __setattr__
    super(BaseDocument, self).__setattr__(name, value)
AttributeError: can't set attribute
"""