Mongoose array bug

bug.js

var mongoose = require('mongoose');

var TestSchema = new mongoose.Schema({
  a : Array,
  s : String,
  n : Number
});
var Test = mongoose.model('Test', TestSchema);

mongoose.connection.once('open', function() {
  this.db.collection('tests', function(err, collection) {
    // First we drop the collection
    collection.drop(function(err) {
      // Then we insert an empty doc
      collection.insert({}, function(err) {
        // And find it with mongoose
        Test.findOne({}, function(err, item) {
          console.log(err, item);
          item.a = undefined;
          item.s = undefined;
          item.n = undefined;
          console.log(item._delta());
        });
      });
    });
  });
});

mongoose.connect('mongodb://127.0.0.1:27017/test');