Ember: set property on resolve promise

I just upgraded ember from 1.11.1 to 1.13.2, and ember-data to 1.13.4

Since then, I'm not able to set a property on a resolved promise:

var promise = store.filter('user', {filters: {"googleId":content.id}}, function() {
        return promise;
 }).then(function(response) {
      var user = response.get('content.0');

      console.log(user); //--> A user entity (Object { type: makeCtor/Class(), id: "38", store: Object, [...])

      user.set('foo', "bar"); // CRASH HERE
 }, function(){ [...] });

The error message from the console is no help, since I'm in a .then()