data does not exist :-(

Hi,

Finally, i've come to the following solution for my stored procedure :
-----------------------------------
-- Function: "SP_U_001"("TypeOfArticle" "varchar")

-- DROP FUNCTION "SP_U_001"("TypeOfArticle" "varchar");

CREATE OR REPLACE FUNCTION "SP_U_001"("TypeOfArticle" "varchar")
  RETURNS SETOF active_articles AS
$BODY$
DECLARE
    myrec RECORD;
    res active_articles;
/**************************************/
BEGIN

  FOR myrec IN
    select *
    from articles, articletypes, department
    where
        articletypes.articletype_type = $1
    AND articles.articletype_id = articletypes.articletype_id
    AND articles.department_id = department.department_id
    AND articles.validity_period_end > now()
  LOOP
    IF (myrec IS NOT NULL) THEN
        res.article_type := myrec.articletypes.articletype_type;
        res.article_author := myrec.articles.author;
        res.department_owner := myrec.department.department_name;
        res.department_picture := myrec.department.department_picture;
        res.article_title := myrec.articles.title;
        res.article_content := myrec.articles.content;
        res.date_creation := myrec.articles.creation_date;
        res.date_start := myrec.articles.validity_period_start;
        res.date_end := myrec.articles.validity_period_end;
    END IF;
      RETURN NEXT res;
    END LOOP;
END;
$BODY$
  LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION "SP_U_001"("TypeOfArticle" "varchar") OWNER TO immensesk;
GRANT EXECUTE ON FUNCTION "SP_U_001"("TypeOfArticle" "varchar") TO immensesk;

-----------------------------------
however, when i run it thanks --> select SP_U_001('action');
i get the following answer : ERROR:  function sp_u_001("unknown") does not exist
in pgAdmin III GUI application

But this function exists.

So where is my mistake ?

thx,

Al.