MongoKit Nested Structure Save Error

[deng....@gmail.com]

Based on the MongoKit documentation, I define a nested structure use dictionary. But when saving the object, I got an error. Details are as below. Can you please kindly advise? Thanks class Swap(Document): structure = { 'coupon': float 'start': int, 'end': int, 'bdc': { 'start': basestring, 'payment': basestring } } And the error I got is: bson.errors.InvalidDocument: Cannot encode object: {'start': 'M', 'payment': 'M'}