Cannot read property 'toJSON' of undefined

[lampercy]

const Foo = skygear.Record.extend('foo');
const foo = new Foo();
foo.c = undefined;
skygear.publicDB.save(foo)
>> Uncaught (in promise) TypeError: Cannot read property 'toJSON' of undefined

Should undefined be casted to null, or warn them skygear does not support undefined?

[rickmak]

We should provide a better error message.