Mongoose Error: `{"message":"No matching document found.","name":"VersionError"}`

I've been trying to figure out why my HTTP Put has not been working after I use it once. When I click a button, I push the current user's id into an array like so:

$scope.currentUser = {
  'eventsAttending' = [];
}

$scope.attending = function(event, id){

  if($cookieStore.get('token')){
    $scope.currentUser = Auth.getCurrentUser();
  }
  $scope.currentUser.eventsAttending.push(event._id);
  $http.put('/api/users/' + $scope.currentUser._id, $scope.currentUser)
    .success(function(data){
      console.log("Success. User " + $scope.currentUser.name);
    });
}

And my HTTP Put function is like so:

var express = require('express');
var controller = require('./user.controller');
var config = require('../../config/environment');
var auth = require('../../auth/auth.service');
router.get('/:id', controller.getEvents);

var router = express.Router();

exports.update = function(req, res) {
  if(req.body._id) { delete req.body._id; }
  User.findById(req.params.id, function (err, user) {
    if (err) { return res.send(500, err); }
    if(!user) { return res.send(404); }
    var updated = _.merge(user, req.body);
    updated.markModified('eventsAttending');
    updated.save(function (err) {
      if (err) { return res.send(500, err); }
      return res.json(200, user);
    });
  });
};

In my HTML page I have multiple events I can attend, and each event has the button called Attend where I call $scope.attending and the function is called and the HTTP Put occurs. All of this works for the first event I choose to attend. However, when I click the Attend button for another event, I get an error that says:

{"message":"No matching document found.","name":"VersionError"}

And I have no idea why. The error occurs when I try to do updated.save() in the mongoose call and I get res.send(500, err)

I tried to look at http://aaronheckmann.blogspot.com/2012/06/mongoose-v3-part-1-versioning.html to solve the issue as I did some googling but I keep getting an error that says:

Undefined type at `versionKey` 
Did you try nesting Schemas? You can only nest using refs or arrays.

Upon adding into my schema:

var UserSchema = new Schema({ versionKey: 'myVersionKey', ...

I also tried to change the .save() function into .update() as someone suggested it online but that seemed to give me even more errors. Any ideas how to fix this? That would be much appreciated!