How is it possible for the repository parameter to be null?

Question

I'm trying to enhance an old project built with symfony 3.2 but i'm facing a weird issue. I'm just trying to get some data in my controller to send it to the view. But, for some reason i can't reach the repository function i made from my controller. Here is the code:

The Repository

class CommandeRepository extends EntityRepository
{
   ...

   public function getSPSExpe()
   {
     $dql = 'SELECT .....(not showing it for security reason)';

     $em = $this->getDoctrine()->getManager('default');
     $query = $em->createQuery($dql);
     $query->setParameters(array(
        'idSite' => 201,
        'idEtat' => 9
     ));
     $results = $query->getResult();
     return $results;
   }
}

The Controller

class ExpeditionController extends Controller
{
    ...

    public function expeditionSPS(CommandeRepository $repo)
    {
       $results = $repo->getSPSExpe();

       return $this->render('WorkflowFrontOfficeBundle:Expedition:expeditionSPS.html.twig', 
       ['results' => $results]);
    }

}

It seems fine right??! but i get this error : Controller ... requires that you provide a value for the "$repo" argument. Either the argument is nullable and no null value has been provided, no default value has been provided or because there is a non optional argument after this one

And when i set CommandeRepository $repo = null as a parameter i get: calling getSPSExpe() on null ....

Do you have any idea about what should be done here? Thanks a lot for your time.

Thanks to Delboy i looked a bit in service container of older symfony versions. I executed "php bin/console debug:container repository and got:

    Information for Service "doctrine.orm.container_repository_factory"
===================================================================

 ------------------ ---------------------------------------------------------------------- 
  Option             Value                                                                 
 ------------------ ---------------------------------------------------------------------- 
  Service ID         doctrine.orm.container_repository_factory                             
  Class              Doctrine\Bundle\DoctrineBundle\Repository\ContainerRepositoryFactory  
  Tags               -                                                                     
  Public             no                                                                    
  Synthetic          no                                                                    
  Lazy               no                                                                    
  Shared             yes                                                                   
  Abstract           no                                                                    
  Autowired          no                                                                    
  Autowiring Types   -                                                                     
 ------------------ ---------------------------------------------------------------------- 

Couldn't find yet how to set autowired to yes for this class.