I have the following java model class in AppEngine: public class Xyz ... { @Persistent private Set<Long> uvw; } When saving an object Xyz with an empty set uvw in Java, I get a "null" field (as listed in the appengine datastore viewer). When I try to load the same object in python (through remote_api), as defined by the following python model class: class Xys(db.Model): uvw = db.ListProperty(int) I get a "BadValueError: Property uvw is required". When saving another object of the same class in python with an empty uvw list, the datastore viewer print a "missing" field. Apparently empty lists storage handling differs between Java and python and lead to "incompatible" objects. Thus my question: Is there a way to, either: force Java to store an empty list as a "missing" field, force Python to gracefully accept a "null" list as an empty list when loading the object? Or any other suggestion on how to handle empty list field in both languages. Thanks for your answers!