Virtual fields with Cakephp 3

I need to have a virtual property in my user entity. I followed the CakePHP book.

UserEntity.php

namespace AppModelEntity;

use CakeORMEntity;

class User extends Entity {

    protected $_virtual = ['full_name'];

    protected function _getFullName() {
        return $this->_properties['firstname'] . ' ' . $this->_properties['lastname'];
    }
}

In a controller

$users = TableRegistry::get('Users');
$user = $users->get(29);
$firstname = $user->firstname; // $firstname: "John"
$lastname = $user->lastname; // $lastname: "Doe"
$value = $user->full_name; // $value: null

I followed exactly the book and I only get a null value.

Answer

According to @ndm, the problem was due to a bad file naming. I named the user entity class UserEntity.php. The CakePHP name conventions says that:

The Entity class OptionValue would be found in a file named OptionValue.php.

Thanks.