Ember 2.15: Change RESTSerializer primaryKey

I have the following serializer:

import DS from 'ember-data';

export default DS.RESTSerializer.extend({

  primaryKey: '_id'

});

The response I get from a .save() is:

{"name":"test","_id":"59df5f7b08e0f5286ead6db1"}

Yet, I still get the error

Error: Assertion Failed: Your person record was saved to the server, but the response does not have an id and no id has been set client side. Records must have ids. Please update the server response to provide an id in the response or generate the id on the client side either before saving the record or while normalizing the response.

What more do I need to do to make this work?