first() returns no result with certain data structures

This one took me a while to track down. It's fairly mysterious, as it only shows up with very particular data structures. Below is a quick script that you can run in the python shell as an example:

import mongoengine
import uuid

mongoengine.connect("test-db")

class TestDocument(mongoengine.Document):
    tech = mongoengine.fields.UUIDField(required=True, unique_with='name', binary=False)
    name = mongoengine.fields.StringField(required=True)
    public = mongoengine.fields.BooleanField(required=True, default=True)
    definition = mongoengine.fields.DictField(required=True)
    default = mongoengine.fields.DynamicField(required=True)

data = {
    'tech': uuid.UUID('ec3d9a97-bee0-4218-88ee-8e500ae7b11e'),
    'name': 'tools',
    'definition': {
        'type': 'object'
    }, 
    'default': {
        "name": "PLA",
        "print_temperature_c": 220,
        "bed_temperature_c": 120,
    }
}

ta = TestDocument(**data)
ta.save()
print TestDocument.objects(**data).count()
print TestDocument.objects(**data).all()
print TestDocument.objects(**data).first()

The issue is that count() returns 1, all() returns a list of length 1, and first returns None. I looked into the source, and I believe it may be related to BaseQuerySet.__getitem__, but I got stymied around line 150.

I would classify this as a fairly urgent bug, as it's a public part of the API that fails silently and mysteriously.

Thanks for the great wrapper by the way - interfaces with Django pretty well.