Patch request fails, why?

SITUATION:

It seems I must have made a mistake in my Mongoose Model or in one of the parameters that are passed to the route.

I am fairly new to the angular2 architecture, so the mistake might be quite obvious.

ERROR:

  ERROR: ValidationError: CastError: Cast to ObjectID failed for value "{ title: 'das',
      username: 'John',
      choice1: 'FSDAFASDF',
      choice2: 'FDSAFD',
      counter1: 11,
      counter2: 0,
      pollId: '5920598ade7567001170c810',
      userId: '591c15b3ebbd170aa07cd476' }" at path "poll"

---

CODE:

route

router.patch('/', function (req, res, next) {
    var decoded = jwt.decode(req.query.token);
    User.findById(decoded.user._id, function (err, user) {
      user.votes = req.body.votes;
      user.save(function(err, result) {
          if (err) {
              console.log("ERROR: "+err);
              return res.status(500).json({
                  title: 'An error occurred',
                  error: err
              });
          }
          res.status(201).json({
              poll: 'Vote Saved',
              obj: result
          });
      });
   });
});

models/user:

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var mongooseUniqueValidator = require('mongoose-unique-validator');

var schema = new Schema({
    firstName: {type: String, required: true},
    lastName: {type: String, required: true},
    password: {type: String, required: true},
    email: {type: String, required: true, unique: true},
    polls: [{type: Schema.Types.ObjectId, ref: 'Poll'}],
    votes: [{
      poll: {type: Schema.Types.ObjectId, ref: 'Poll'},
      choice: {type: Number},
    }],
});

schema.plugin(mongooseUniqueValidator);

module.exports = mongoose.model('User', schema);

models/poll

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

var User = require('./user');

var schema = new Schema({
    title: {type: String, required: true},
    choice1: {type: String, required: true},
    choice2: {type: String, required: true},
    counter1: {type: Number, required: true},
    counter2: {type: Number, required: true},
    user: {type: Schema.Types.ObjectId, ref: 'User'}
});

schema.post('remove', function (poll) {
    User.findById(poll.user, function (err, user) {
        user.polls.pull(poll);
        user.save();
    });
});

module.exports = mongoose.model('Poll', schema);

via Coder1000