Cannot save a record using its relationship

Hello,

Given the following two very basic models:

class User extends \Phalcon\Mvc\Model {
    public $id;
    public $password;

    public function initialize() {
        $this->hasOne('id', 'Person', 'user_id', [
            'alias' => 'corresponding_person'
        ]);
    }
}
// CREATE TABLE User (
//   id int(10) NOT NULL AUTO_INCREMENT,
//   password VARCHAR(30),
//   PRIMARY KEY (id)
// );

class Person extends \Phalcon\Mvc\Model {
    public $id;
    public $user_id;
    public $name;

    public function initialize() {
        $this->hasOne('user_id', 'User', 'id', [
            'alias' => 'corresponding_user'
        ]);
    }
}
// CREATE TABLE Person (
//   id int(10) NOT NULL AUTO_INCREMENT,       
//   user_id int(10) NOT NULL,
//   name VARCHAR(30),
//   PRIMARY KEY (id)
// );

and this controller:

class IndexController extends ControllerBase {

    public function indexAction() {
        $User = new User();
        $User->password = 'secret';
        $User->save();

        $Person = new Person();
        $Person->name = 'John';
        $Person->corresponding_user = $User;

        if (! $Person->save()) {
            die(
                'Messages : ' . print_r($Person->getMessages(), true) . "\n" . 
                '$User->id : ' . $User->id
            );
        }
    }
}

When I run the controller, I get:

Messages : Array
(
    [0] => Phalcon\Mvc\Model\Message Object
        (
            [_type:protected] => PresenceOf
            [_message:protected] => user_id is required
            [_field:protected] => user_id
            [_model:protected] => 
            [_code:protected] => 0
        )
)
$User->id : 4

$User is saved, but not $Person.

It works if I use $Person->user_id = $User->id; but shouldn' it also work using the relationship defined in the model?

(Note: removing the relationship aliases and using $Person->User = $User produces the same error)

Thanks in advance for any help