Request hangs forever when saving document with references, callback never executes

[ch...@bizpub36.com]

I have a schema that looks like this:

var ZoneSchema = new Schema({
    name: String,
    coupons: [{ type: Schema.Types.ObjectId, ref: 'Coupon' }],
    magazines: [{ type: Schema.Types.ObjectId, ref: 'Magazine' }]
});

mongoose.model('Zone', ZoneSchema);

I have a request to add a new zone with the coupon and magazine arrays empty initially. The callback in the save function never executes and the request hangs forever until the client just gets a 502 error, but the server logs a 200 and doesn't crash or show any errors. The document is never saved.

exports.add = function(req, res) {
    var newZone = new Zone({
        name: req.body.name
    });

    //console.log here works fine
    console.log('something');

    newZone.save(function(err) {
        //console.log here is never executed
        console.log('something else');

        if(!err) {
            res.send({ status: 'ok', message: 'success' });
        }
        else {
            res.send(err);
        }
    });
}

save() works fine on models without references though.