mysql y control de flujo. ayuda

[rodrigo_lopez]

hice este:

CREATE procedure check_themes(IN id int)
BEGIN
DECLARE tema CHAR(1);
SELECT predeterminado INTO tema FROM themes WHERE id_theme=id;
CASE tema
WHEN '1' UPDATE themes set predeterminado = '1' where id_theme = 1;
END CASE;
END$$

y me da este error:
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UPDATE themes set predeterminado = '1' where id_theme = 1;
END CASE;
END' at line 6

puse el delimiter lo del $$. que puede ser?