Assertion Failed: You attempted to access the size property

[Turbo87]

on Ember 3.1.0-beta.1:

Assertion Failed: You attempted to access the size property (of DS.Errors:ember1800).
Since Ember 3.1, this is usually fine as you no longer need to use .get()
to access computed properties. However, in this case, the object in question
is a special kind of Ember object (a proxy). Therefore, it is still necessary
to use .get('size') in this case.

If you encountered this error because of third-party code that you don't control,
there is more information at #16148, and
you can help us improve this error message by telling us more about what happened in
this situation.

This seems to be caused by Ember Data using isEmpty() on a DS.Errors object. Since isEmpty() checks obj.size unconditionally it is causing the error above.

[Turbo87]

Seems to be related to #9414

/cc @mixonic

[rwjblue]

This was fixed in emberjs/data#5340. The issue is that the DS.Errors unknownProperty returned null instead of undefined (and our proxy assertion only “allows” undefined).