Mongoose: Updating values in null object results in mongo 10145 LEFT_SUBFIELD

gistfile1.js

var mongoose = require('mongoose'),
	_ = require('underscore');

mongoose.connect('mongodb://localhost/test');
mongoose.set('debug', true);

var docSchema = new mongoose.Schema({
	title: String,
	meta: {
		enabled: Boolean,
		amount: Number,
		note: String
	}
});

var Doc = mongoose.model('docs', docSchema);

function findAndSaveMeta() {
	Doc.findOne({ title: "Test" }, function(err, doc){
		doc.meta = { enabled: true, amount: 1235, note: null };

		/*
			//Setting mark modified will fix this issue.
			doc.markModified('meta');
		*/

		doc.save(function(err, doc){
			console.log(err); // Gives LEFT_SUBFIELD only supports Object
			Doc.remove(); // Remove test docs
		});
	});
}

function findAndSaveNull() {
	Doc.findOne({ title: "Test" }, function(err, doc){
		doc.meta = null;

		doc.save(function(err, doc){
			findAndSaveMeta();
		});
	});
}

var testSave = new Doc({ title: "Test", meta:null });

testSave.save(function(err, doc){
	findAndSaveNull();
});