Why does Mongoose model throw "TypeError: Object { _id: x } has no method 'y'"?

Hey I am new to mongo and mongoose. I tried upating a specific document by _id but I keep on getting an error

TypeError: Object { _id: 4fd02c1d50071a5713000001 } has no method 'update'. 

My code is as follows:

//Update comment or increment vote up or vote down
app.put('/comments/voteUp/:commentid', function(request, response){

var id = mongoose.Types.ObjectId(request.params.commentid);

var conditions = { "_id": request.params.commentid }
  , update =  {$inc: { 'meta.voteUp': 1 } }
  , options = { multi: false };

console.log(conditions);
var comment = new CommentModel();
comment.update( { "_id": id, update, options, callback );

function callback (err, numAffected) {
    response.send("numAffected: " + numAffected);
}
console.log("commentid: " + request.params.commentid);
  });

Any assistance will be greatly appreciated