symfony 2.1 injection doctrine in service - symfony

Question

I need to get some data in my db and the current user, so I use the entity manager and security.context in my service
I have this error :
Fatal error: Call to a member function getRepository() on a non-object
in path/to/file on line 84
service.yml :
services:
ns_messagerie.letterboxcore:
class: ns\MessagerieBundle\LetterBoxCore\LetterBoxCore
arguments: [#security.context, #doctrine.orm.entity_manager]
Dependency injection :
class nsMessagerieExtension extends Extension
{
/**
* {#inheritDoc}
*/
public function load(array $configs, ContainerBuilder $container)
{
$configuration = new Configuration();
$config = $this->processConfiguration($configuration, $configs);
$loader = new Loader\YamlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
$loader->load('services.yml');
}
}
And my service :
class LetterBoxCore {
protected $securityContext;
protected $em;
public function __construct( $securityContext, $entityManager) {
$this->securityContext = $securityContext;
$this->em = $entityManager;
}
public function countNbNotRead(Utilisateur $user = null, Discussion $discussions) {
//...
}
public function getAllDiscussion(Utilisateur $user = null, $all = null) {
// line 84:
$list = $em->getRepository('nsMessagerieBundle:ParticipantMessagerie')
->findBy(array('participant' => $user,
'supprimer' => $all
)
);
}
public function getBAL(Utilisateur $user = null) {
// Call the method countNbNotRead and GetAllDiscussion
}