Mongoose add ref in array

I have this mongoose schema :

var BookSchema = new mongoose.Schema({
    title: {
        type: String, required: true
    },
    description: {
        type: String, required: true
    },
    questions:[{
        _id : {type: mongoose.Schema.ObjectId, ref: 'Question'},
        value: {type: Number, default: -1}
    }]
});

And QuestionSchema :

var QuestionSchema = new mongoose.Schema({
   title: {
       type: String, required: true, unique: true
   }
});

I'm trying to add "questions" in the array but I have a "CastError" when I do :

    Book.update({'_id': req.params.id}, {$addToSet: {'questions._id': tabQuestions}}, function(err) {
if (err)
  console.log(err);
});

Th array "tabQuestions" is an array with string which are the ObjectID of a Question. For exemple : ["551950a8a0927a741c6c1d46","551950c0a0927a741c6c1d47"]

I don't understand why I have this error :

Cast to ObjectId failed for value \"551950a8a0927a741c6c1d46,551950c0a0927a741c6c1d47\" at path \"_id\""