when a subdoc of Mongoengine Document has no default setted

when a subdoc of Mongoengine Document has no default setted, but the form has data for this sub-doc, it will raise error.

testcase.
class Location(EmbeddedDocument):
    address = StringField(max_length=128, required=True)
    city = StringField(max_length=128)
    state = StringField(max_length=128)
    zip = StringField(max_length=16)
    lat = StringField(max_length=32)
    lon = StringField(max_length=32)

class School(Document):
    name = StringField(max_length=128, required=True)
    location = EmbeddedDocumentField(Location)


when form post 'location-address' : 'aaaa', the error raised.