Error! { [CastError: Cast to ObjectId failed for value at path "_id"]

I was following http://mongoosejs.com/docs/populate.html to learn how to populate an array of references I had. However, when I try to perform an HTTP Get request on the populated array, I get an error that says

Error! { [CastError: Cast to ObjectId failed for value "blah" at path "_id"]
  message: 'Cast to ObjectId failed for value "blah" at path "_id"',
  name: 'CastError',
  type: 'ObjectId',
  value: 'blah',
  path: '_id' }

With blah being the value inside the array. Below is my attempt:

HTTP Get call:

$scope.user = Auth.getCurrentUser();

$http.get('/api/users/' + $scope.user._id)
    .success(function(data, status, headers, config){
        $scope.currentUser = data;
    });

User Schema:

var UserSchema = new Schema({
  name: String,
  username: String,
  eventsAttending: [{ type: String, ref: 'Event'}],
});

and HTTP GET function:

router.get('/:id', controller.getEvents);
exports.getEvents = function(req, res) {
  User.findById(req.params.id).populate('eventsAttending').exec(function(err, events){
    if(err) {
      console.log("Error!", err);
      return res.send(500, err);
    }
    return res.json(200, events);
  });
};

I honestly am not too sure why the values inside my array are being casted into Object Ids. I clearly specified that the contents in the array are Strings, not Object Id's which will be references to the the Event Schema. I know that the contents inside my array are simple Strings. Any idea why this is happening?

Edit: I am posting a sample object. In here lies the eventsAttending array I want to populate:

{
  "name" : "Jane Doe",
  "username" : "jane93",
  "eventsAttending" : [
      "blah",
      "TestEvent",
      "Asdf",
      "TestEvent2"
  ]
}