Cannot read property scope of undefined

TypeError: Cannot read property 'scope' of undefined
    at EmbeddedDocument.set [as votes] (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/document.js:1915:38)
    at _createConstructor (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/schema/documentarray.js:81:35)
    at new DocumentArray (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/schema/documentarray.js:29:26)
    at Function.Schema.interpretAsType (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/schema.js:581:14)
    at Schema.path (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/schema.js:526:29)
    at Schema.add (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/schema.js:407:12)
    at new Schema (/home/pc/Desktop/stackoverflow-sample-rest/node_modules/mongoose/lib/schema.js:100:10)
    at Object.<anonymous> (/home/pc/Desktop/stackoverflow-sample-rest/models.js:43:22)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.require (module.js:497:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/home/pc/Desktop/stackoverflow-sample-rest/routes.js:5:35)

'use strict';

var mongoose = require('mongoose');

var Schema = mongoose.Schema;

var sortAnswers = function (a,b) {
  // negative if a before b
  // 0 to leave the order
  // positive if a is after b based on votes
  if(a.votes === b.votes) {
    // If vote counts same, order by latest first.
    return b.updatedAt - a.updatedAt;
  }
  return b.votes - a.votes;
};

// Define AnswerSchema before QuestionSchema.
// Mongo has a document size limit of 16MB so if the children document is going to grow too big then it is advisable to store them in a separate collection referencing using ObjectId
var AnswerSchema = new Schema({
  text: String,
  createdAt: {type: Date, default: Date.now},
  updatedAt: {type: Date, default: Date.now},
  votes: {type: Number, default: 0}
});

// Mongoose way of defining instance methods
AnswerSchema.method("update", function (updates, callback) {
  Object.assign(this, updates, {updatedAt: new Date()});
  this.parent().save(callback);
});


AnswerSchema.method("votes", function (vote, callback) {
  if(vote === "up") {
    this.votes += 1;
  } else {
    this.votes -= 1;
  }
  this.parent().save(callback);
});

var QuestionSchema = new Schema({
  text: String,
  createdAt: {type: Date, default: Date.now},
  answers: [AnswerSchema],
});

QuestionSchema.pre("save", function (next) {
  this.answers.sort(sortAnswers);
  next();
})

var Question = mongoose.model("Question", QuestionSchema);

module.exports.Question = Question;

'use strict';

var express = require('express');
var router = express.Router();
var Question = require('./models').Question;

// param route handler.
router.param('qID', function (req, res, next, id) {
  Question.findById(req.params.qID, function (err, doc, next) {
    if(err) return next(err);
    if(!doc) {
      err = new Error('Not Found');
      err.status = 404;
      return next(err);
    }
    req.question = doc;
    return next();
  });
});

router.param('aID', function (req, res, next, id) {
  req.answer = req.question.answers.id(id);
  if(!req.answer) {
    err = new Error('Not Found');
    err.status = 404;
    return next(err);
  }
  return next();
})

// This will create route to /questions/
router.get('/', function (req, res, next) {
  // Return all questions.
  Question.find({})
    .sort ({createdAt: -1})
    .exec(function (err, questions) {
    if(err) return next(err);
    res.json(questions);
  });
});

// Check with postman /.POST http://localhost:3000/questions
router.post("/", function (req, res) {
  var question = new Question(req.body);
  question.save(function (err, question, next) {
    if(err) return next(err);
    res.status(201);
    res.json(question);
  });
});

// GET particular question
// check with postman /GET http://localhost:3000/questions:2
router.get('/:qID', function (req, res, next) {
  res.json(req.question);
});

// POST /questions/:id/answers
router.post("/:qID/answers", function (req, res, next) {
  req.question.answers.push(req.body);
  req.question.save(function (err, question) {
    if(err) return next(err);
    res.status(201);
    res.json(question);
  });
});

// PUT /questions/:qID/answers/:aID
router.put('/:qID/answers/:aID', function (req, res) {
  req.answer.update(req.body, function (err, result) {
    if(err) return next(err);
    res.json(result);
  });
});

// DELETE /questions/:qID/answers/:aID
router.delete('/:qID/answers/:aID', function (req, res) {
  req.answer.remove(function (err) {
    req.question.save(function (err, question) {
      if(err) return next(err);
      res.json(question);
    });
  });
});

// POST /questions/:qID/answers/:aID/vote-up
/// POST /questions/:qID/answers/:aID/vote-down
// Express can handle any number of middleware and it will execute them sequentially.
// Here, I have added second middleware to make sure correct parameter is passed to dir parameter.
router.post('/:qID/answers/:aID/vote-:dir',
function (req, res) {
  if(req.params.dir.search(/^up|down$/) === -1) {
    var err = new Error("Not Found");
    err.status = 404;
    // If error call next with parameter.
    next(err);
  } else {
    // else call next without parameter.
    req.vote = req.params.dir;
    next();
  }
},
function (req, res, next) {
  req.answer.vote(req.vote, function (err, question) {
    if(err) return next(err);
    res.json(question);
  });
});

module.exports = router;