Mongoose error due to incorrect schema statement: MissingSchemaError: Schema hasn't been registered for model "undefined".

facepalm.js

#!/usr/bin/env node

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

var catSchema = new Schema({
  name: String
});
var Cat = mongoose.model('Cat', catSchema);

var personSchema = Schema({
  name: String
  , cats: {type: [Schema.Types.ObjectId], ref: 'Cat'}
  //             ^                     ^
  //                    should be:
  //, cats: [{type: Schema.Types.ObjectId, ref: 'Cat'}]
  //        ^                                         ^
});
var Person = mongoose.model('Person', personSchema);

mongoose.connect('mongodb://localhost/mongoose-schema-facepalm');
Cat.create({name: 'fluffy'}, function(err, cat){
  Person.create({name: 'bob', cats: [cat]}, function(err, person){
    Person.findById(person.id).populate('cats').exec(function(err, person){
      console.log(person.cats[0].name);
    });
  });
});

// results in:
// /home/efoster/work/facepalm/node_modules/mongoose/lib/utils.js:397
//        throw err;
//              ^
// MissingSchemaError: Schema hasn't been registered for model "undefined".