Novice with Postgresql - trying simple Stored Procedure

I have a simple 3 column table. The Table is a Month Name to number table
which also includes an effective date column. So 12 rows, 3 columns each.

Here is the Stored Procedure I constructed to get the number if given the
name (3 parameters for the SP):

--/
DROP PROCEDURE month_lookup
(in mth_name TEXT,
inout mth_num TEXT,
inout ret_cd TEXT);
CREATE OR REPLACE PROCEDURE month_lookup
(in mth_name TEXT,
inout mth_num TEXT,
inout ret_cd TEXT default '00000^00')
LANGUAGE plpgsql AS $$
BEGIN
-- SET mth_num = num WHERE name = mth_name;
SELECT
DISTINCT month_nm_2_num.month_nm_2_num.num
FROM
month_nm_2_num.month_nm_2_num
WHERE
month_nm_2_num.month_nm_2_num.name = mth_name
ORDER BY
month_nm_2_num.month_nm_2_num.name ASC
INTO mth_num;
RAISE NOTICE '%', mth_num;
RETURN;
END; $$;
/

Here is the invocation of that SP:

--/
DO $$
DECLARE
mth_name TEXT;
ret_cd TEXT;
mth_num TEXT;
BEGIN
call month_lookup ('Jan','00',null);
/* raise notice '%', mth_num;*/
END $$;
/

Here is the error I am receiving (using DB-Visualizer):

I have tried a lot of different things. I have researched a number of
examples and I am stumped. I am fairly certain it is something simple and
related to the parameter in the SP.

Can anybody straighten me out?

Thanks in advance,

Jim Kosloskey