nestedvalidation.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var assert = require('assert')
mongoose.connect('localhost', 'testing_findone');

var schema = new Schema({
  name: { last: { type: String, required: true} }
});

var A = mongoose.model('A', schema);

mongoose.connection.on('open', function () {
  A.findOne({}, ['name'], function (err, doc) {
    if (err) console.error(err.stack||err);
    assert.equal(null, doc);
    console.error('all good');

    mongoose.connection.db.dropDatabase(function () {
      mongoose.connection.close();
    });
  });
});

please open a ticket on the repo. :)