Mongodb duplicate key error collection dup key: null

Im trying to create multiple accounts.

The first account always works, but when im trying to make a new one i get the following error:

BulkWriteError: insertDocument :: caused by :: 11000 E11000 duplicate key error index: db.users.$friends.userid_1  dup key: { : null }

The first user is fine, containing friends with an empty array, as i would like it to be.

But the next user is not created.

What do i have to do to fix this error?

The user schema piece of friends in users is the following:

   friends : [
    {
        userid : {type: String, default: '', unique: true },
    }
],
friendRequests: [
    {
        userid : {type: String, default: '', unique: true },
    }

EDIT:

i have been looking into https://docs.mongodb.com/manual/core/index-unique/#unique-index-and-missing-field but still couldnt make it work.

EDIT2:

By default, its not creating any friends or friendrequests.

EDIT3:

full code:

passport.use('local-signup', new LocalStrategy({
            usernameField : 'username',
            passwordField : 'password',
            passReqToCallback : true,
        },
        function(req, username, password, done) {
            process.nextTick(function() {
                console.log("doing local signup");
                username = username.toLowerCase();
                Account.findOne({username :  username }, function(err, user) {
                    var expr = "/admin/";
                    if (err) {
                        return done(err);
                    } else if (user) {
                        return done(null, false, 'That username is already taken.');
                    } else if(username.length < 3 || username.length >= 12) {
                        return done(null, false, 'Username has to be between 3 and 12 characters! :( '  + username);
                    } else if(/^[a-zA-Z0-9- ]*$/.test(username) == false) {
                        return done(null, false, 'You cant have any special characters!');
                    } else if(password.length < 5 || password.length > 15) {
                        return done(null, false, 'Password need to be 5-15 characters long!');
                    } else {

                        var newUser            = new Account();
                        newUser.username    = username;
                        newUser.password = newUser.encryptPassword(password);
                        newUser.save(function(err) {
                            if (err)
                                throw err;
                            return done(null, newUser);
                        });
                    }
                });

            });

        }));

user model:

var mongoose     = require('mongoose');
var Schema       = mongoose.Schema;
var passportLocalMongoose = require('passport-local-mongoose');
var bcrypt   = require('bcrypt-nodejs');



var UserSchema   = new Schema({
    username: {type: String, index: { unique: true }},
    password: {type: String},
    salt: { type: String},
    hash: {type: String},
    gender : {type: String, default: 'male'},
    friends : [
        {
            userid : {type: String, default: '', unique: true },
        }
    ],
    friendRequests: [
        {
            userid : {type: String, default: '', unique: true },
        }
    ]

});
UserSchema.methods.encryptPassword = function(password) {
    return bcrypt.hashSync(password, bcrypt.genSaltSync(10));
}

UserSchema.methods.validPassword = function(password) {
    return bcrypt.compareSync(password, this.password);
}



module.exports = mongoose.model('Users', UserSchema);