set d'une var qui retourne null

salut a vous les developpeurs !
voila, j'ai un souci lors de l'appel a une procedure stockée que voici : elle retourne la valeur null pour @id_pers!

merci de votre aide
CREATE PROCEDURE SelectRoleParForm  @id_ss_serv bigint, @id_form bigint, @wwid_pers varchar(50) AS
 declare @id_pers bigint
 set @id_pers=(select id_pers from personne where wwid_pers=@wwid_pers)
 if(@id_ss_serv = 0)
 begin
  select distinct *
  from formulaire, proposer_role, role, etre_attribue
  where formulaire.id_form=proposer_role.id_form AND proposer_role.id_role=role.id_role AND date_annul_role='01/01/1900' AND formulaire.id_form=@id_form
  AND role.id_role NOT IN (select id_role from etre_attribue where id_pers=@id_pers)
 end 
 else
 begin
  select distinct *
  from formulaire, proposer_role, role, sous_service, etre_attribue
  where formulaire.id_form=proposer_role.id_form AND proposer_role.id_role=role.id_role  AND date_annul_role='01/01/1900' AND formulaire.id_form=@id_form AND sous_service.id_ss_serv=role.id_ss_serv
AND role.id_role NOT IN (select id_role from etre_attribue where id_pers=@id_pers)
AND sous_service.id_ss_serv=@id_ss_serv
 end
GO