Setter isn't invoked when value === null

setter-test.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var assert = require('assert');

mongoose.connect('mongodb://localhost/test');


var OrderSchema = new Schema({
  total: {
    type: Number,
    default: 0,
    set: function (value) {
      return (value || 0) + 10;
    }
  }
});


var Order = mongoose.model('offer', OrderSchema);

var TEST_VALUE = null; // Works fine when value !== null

var order = new Order({
  total: TEST_VALUE
});

order.save(function (err, order) {
  if (err) {
    return console.error(err.message);
    process.exit(1);
  }

  assert.equal(order.total, TEST_VALUE + 10, 'Setter is run');

  process.exit(0);
});