Mongoose: Using the same schema in two separate arrays - node.js

Question

I have two schemas, defined as following:
var userSchema = new Schema({
email: String,
name: String,
role: String,
password: String
})
var raceSchema = new Schema({
date: Date,
name: String,
location: String,
time: String,
register: String,
participants: [{ type: Schema.Types.ObjectId, ref: 'User'}],
registered_participants: [{ type: Schema.Types.ObjectId, ref: 'User'}],
})
As you can see, I reference the first schema twice in the second schema. If I add a reference to a user in one of the lists, everything is fine. But when I add a reference to the same user to the other list I get the following error: Cast to [undefined] failed for value
What causes this error? Is it related to the fact that the same schema is used twice in the second schema?
Edit:
I get the error when I call the following Express endpoint:
app.post('/race/:id/registered', passport.authenticate('jwt', { session: false}), (req, res) =>
Race.findOne({ _id: req.params.id }, function (err, race) {
if (err) return res.json({'Error': err})
if (!race) return res.json({'Error': 'Race not found'})
race.registered_participants.push(req.user)
race.save(function (err, updatedRace) {
if (err) return res.json({'Error': err})
res.send(updatedRace)
})
})
)
Edit 2: The model definitions:
var User = mongoose.model('User', userSchema);
var Race = mongoose.model('Race', raceSchema);

Answer 1

Try using findByIdAndUpdate in your POST method instead:
app.post('/race/:id/registered', passport.authenticate('jwt', { session: false}), (req, res) =>
Race.findByIdAndUpdate(req.params.id,
{ $push: { registered_participants: req.user } },
function (err, race) {
if (err) return res.json({'Error': err})
res.send(race)
})
)