Mongoose + Expressjs - E11000 duplicate key error index?

I don't understand why I got this error below after adding the first entry:

E11000 duplicate key error index: mydb.datasets.$id_1  dup key: { : null }

I don't have any null value in my first entry:

{
    "index" : "9IPZMW7IL",
    "name" : "Tweets",
    "owner_name" : "xxx",
    "read_key" : "fb6f9125f4ca15c33fea89416c3351d1",
    "write_key" : "d8a6c7e5fc73b5a91aa7a533565ed1f1",
    "data" : {
        "var1" : {
            "name" : "particles"
        }
    },
    "_id" : ObjectId("57729dc20cb70952424cdbb4"),
    "created_at" : ISODate("2016-06-28T15:54:42.576Z"),
    "entries_number" : 0,
    "public" : true,
    "__v" : 0
}

Below is my code:

// CRUD API:
// POST/ Create new dataset request
router.post("/", helper.authenticate, function(req, res) {
    // Used to set the dataset owner
    var sessionUser = req.session.user.name;
    // Get values from the post request
    var name = req.body.name;
    var isPublic = req.body.public != undefined ? true:false;
    // Delete the values from the request body so that we only keep information about the variables
    delete req.body.name;
    delete req.body.public;

    // This is so that we can loop through the object in reverse order
    // We do that so that the fields are saved in the right order on the db
    // (this way it will appear in the right order on the 'edit' view)
    var propertiesList = [];
    for (var property in req.body) {
        if (req.body.hasOwnProperty(property)) {
            propertiesList.push(property);
        }
    }
    propertiesList.reverse();

    var variablesFields = {};
    for (var i in propertiesList) {
        console.log(propertiesList[i])
        variablesFields[propertiesList[i]] = {name:req.body[propertiesList[i]],
                                    values: Array};
    }

    // Create dataset
    Dataset.create({
        index: helper.uniqueIndex(),
        name: name,
        owner_name: sessionUser,
        read_key: hat(),
        write_key: hat(),
        public: isPublic,
        data: variablesFields
    }, function(err, dataset) {
        if (err) {
            console.log("Error creating the dataset: " + err);
            req.session.error = "A problem occured when creating the dataset. Please try again.";
        } else {
            console.log("New dataset created with id: " + dataset._id);
            req.session.success = "Dataset " + name + " created successfully.";
        }
        res.redirect("/index");
    });
});

Error:

Error creating the dataset: WriteError({"code":11000,"index":0,"errmsg":"E11000 duplicate key error index: mydb.datasets.$id_1 dup key: { : null }","op":{"index":"2IPZMWHGI","name":"PM 2","owner_name":"xxx","read_key":"fc31c152aa86070252c70c0304e4ca5c","write_key":"238110753c8762ce4a547fa02100a299","data":{"var1":{"name":"particles"}},"_id":"57729dcf0cb70952424cdbb5","created_at":"2016-06-28T15:54:55.459Z","entries_number":0,"public":true,"__v":0}})

Model:

var datasetSchema = new mongoose.Schema({
    index: {type: String, required: true, index: {unique: true}},
    name: {type: String, required: true},
    owner_name: {type: String, required: true},
    read_key: {type: String},
    write_key: {type: String},
    public: {type: Boolean, default: false},
    data: {type: Object},
    entries_number: {type: Number, default: 0},
    created_at: {type: Date, default: Date.now},
    last_entry_at: {type: Date}
});

Any idea why and how I can fix this?