How to validate the "Object" type in Mongoose schemas?

Question

JavaScript

var mongoose = require('mongoose');
var modelSchema = new mongoose.Schema({
  obj: {
    type: Object,
    required: true
  }
});

var Model = mongoose.model('model', modelSchema);
var model = new Model({ obj: 'anything' });
model.save(function(err, doc) { console.log(err, doc); });

// -> null, { obj: 'anything' }

The problem is that we describe in the schema that the property should be an object, but in fact we can write anything there, except for nulland undefined. I understand that in JS everything except nulland undefinedis an object, but I would like some order in MongoDB collections :)
Is there any nice way to validate this thing?