Mongoose validation does not work when using ref

mongoose_validation_test.js

var mongoose = require('mongoose'),
    Schema = mongoose.Schema

mongoose.connect('mongodb://localhost/my_database')

// Define User Schema
var UserSchema = new Schema({
  name: {type: String}
})
var User = mongoose.model('User', UserSchema)

// Define Comment Schema
var CommentSchema = new Schema({
  content: {type:String, required: true},
  actor: { type: Schema.ObjectId, ref: 'User', required: true}
})
var Comment = mongoose.model('Comment', CommentSchema)

// The following test passes (REQUIRED on type String)
exports.testContentRequired = function(test) {
  test.expect(3)
  var user = new User({
    name: 'Joe'
  })
  user.save(function(err, data) {
    var comment = new Comment({
      actor: user
    })
    comment.save(function(err, data) {
      test.ok(err)
      test.equal('Validation failed', err && err.message)
      test.ok(err && err.errors.content)
      test.done()
    })
  })
}

// The following test does not pass (Required when using ref)
exports.testActorRequired = function(test) {
  test.expect(3)
  var user = new User({
    name: 'Joe'
  })
  user.save(function(err, data) {
    var comment = new Comment({
      content: 'My comment goes here'
    })
    comment.save(function(err, data) {
      test.ok(err)
      test.equal('Validation failed', err && err.message)
      test.ok(err && err.errors.actor)
      test.done()
    })
  })
}