PHP - Symfony 4 - API Platform - Custom operation

Using SF4 & Api platform : I would like to have a route for performing a random computation, and returning result to the client.. this seems to be simple
However I am not able to do it easily with custom operations

Here it is what I have done

I have my entity, for request & reponse, using normalization to have field only in request, and not in response
And I have my Controller with my custom operation + the corresponding route only for collection (as I don't have any id) in yaml in the api_platform/resources.yaml file
The controller is taking the entity in input and responding with it, hydrated by the result

I am getting the error :

2018-08-18T16:22:42+02:00 [critical] Uncaught PHP Exception ApiPlatform\Core\Exception\InvalidArgumentException: "Unable to generate an IRI for the item of type "App\Entity\Computation"" at /dev/git/app-api/vendor/api-platform/core/src/Bridge/Symfony/Routing/IriConverter.php line 127

But I think this is when the server is serializing the reponse, as the computation is performed (log inside are printed)

Entity\Computation.php :

class Computation
{
/**
 * @var double  you input
 *
 * @Assert\NotBlank
 * @Groups({"read","write"})
 */
public $input;

/**
 * @var double the result
 *
 * @Groups({"read"})
 */
public $result;
}

Controller\ComputationController.php :

class ComputationController
{
    private $service;

    public function __construct(MyService $service)
    {
        $this->service= $service;
    }

    public function __invoke(Position $data): Response
    {
        $this->service->compute($data);
        return $data;
    }
}

api_platform/resources.yaml :

resources:
  App\Entity\Computation:
    itemOperations: {}
    collectionOperations:
      compute:
        method: 'POST'
        path: '/compute'
        controller: 'App\Controller\ComputationController'
    attributes:
      normalization_context:
        groups: ['read']
      denormalization_context:
        groups: ['write']

Can someone help me with it ?
Thanks !