KeyError in MongoKit when adding nested field to structure

Question

I have the following structure:

structure = {
    'firstname': basestring,
    'lastname': basestring,
    'genres': [basestring],
    'address': [
        {'number': basestring, 'street': basestring, 'town': basestring}
    ],
    'phone': [
        {'type': basestring, 'number': basestring}
    ],
}

And I have a small helper method to iterate over cursors to return a python dict like so:

def to_django_context(cursor):
    records = []
    for r in cursor:
        records.append(r.to_json_type())
    return records

this works fine until I want to add another nested field to the structure like this:

structure = {
    'firstname': basestring,
    'lastname': basestring,
    'genres': [basestring],
    'address': [
        {'number': basestring, 'street': basestring, 'town': basestring}
    ],
    'phone': [
        {'type': basestring, 'number': basestring}
    ],
    'title': [{'TEST_FIELD': basestring}],
}

at which point my cursor iterator fails with a KeyError. If I delete all the documents in the collection it works normally. So does this mean that everytime I change my document structure object I have to essentially drop the collection?

Cheers, F