Unable to add a subdocument to an extended schema

[Liron-K]

When I try and add the subdocument onto the extended schema, I get the following error:

{ [ValidationError: Car validation failed]
message: 'Car validation failed',
name: 'ValidationError',
errors:
{ engine:
{ [CastError: Cast to Embedded failed for value "{ _id: 57738e9cc1e7b9f5fe43ebaf, liters: 1.2 }" at path "engine"]
message: 'Cast to Embedded failed for value "{ _id: 57738e9cc1e7b9f5fe43ebaf, liters: 1.2 }" at path "engine"',
name: 'CastError',
kind: 'Embedded',
value: [Object],
path: 'engine',
reason: [TypeError: this.cast is not a function] } } }

Here is a test that i put together that replicates this.

var EngineSchema = new Schema({
  liters: {
    type:Number,
    default:1.2
  }
}, { safe : true });
var Engine = mongoose.model('Engine', EngineSchema);

var VehicleSchema = new Schema({
  name : String,
  engine: EngineSchema
}, { safe : true });
var Vehicle = mongoose.model('Vehicle', VehicleSchema);

var CarSchema = VehicleSchema.extend({
  doors : {
    type:Number,
    default:2
  }
});
var Car = mongoose.model('Car', CarSchema);

describe('Mongoose', function() {

  //This test succeeds
  it('can save with subdocument', function(done) {
    var vehicle = new Vehicle();
    chai.expect(vehicle.doors).to.equal(undefined);
    console.log("save vehicle");
    vehicle.name = "Name";
    vehicle.save()
    .then(function(savedVehicle){
      chai.expect(savedVehicle.name).to.equal("Name");
      chai.expect(savedVehicle.doors).to.equal(undefined);
      chai.expect(savedVehicle.engine).to.equal(undefined);

      var engine = new Engine();
      chai.expect(engine.liters).to.equal(1.2);

      vehicle.engine = engine;
      return vehicle.save();
    })
    .then(function(savedVehicle){
      chai.expect(savedVehicle.name).to.equal("Name");
      chai.expect(savedVehicle.doors).to.equal(undefined);
      chai.expect(savedVehicle.engine).to.not.equal(undefined);
      chai.expect(savedVehicle.engine.liters).to.equal(1.2);
      done();
    }, function(err){
      console.log(err);
    });
  });

  //This test fails
  it('should extend with subdocument', function(done) {
    var car = new Car();
    chai.expect(car.doors).to.equal(2);
    console.log("save car");
    car.save()
    .then(function(savedCar){
      chai.expect(savedCar.doors).to.equal(2);
      chai.expect(savedCar.engine).to.equal(undefined);

      var engine = new Engine();
      chai.expect(engine.liters).to.equal(1.2);

      car.engine = engine;
      return car.save();
    })
    .then(function(savedCar){
      chai.expect(savedCar.doors).to.equal(2);
      chai.expect(savedCar.engine).to.exist();
      chai.expect(savedCar.engine.liters).to.equal(1.2);

      done();
    }, function(err){
      console.log(err);
    });
  });
});