What should return trigger BEFORE DELETE

I am trying to make simple trigger function (Postgresql) but I am getting same error 'function did not return any row' in all these cases (just simple examples):

New:

UPDATE somewhere SET something = something - 1;
RETURN NEW;

Old:

UPDATE somewhere SET something = something - 1;
RETURN OLD;

What should I return when I call this function "before delete"? ("after insert/update" works well)

Tyvm for tips!

Full code as requested: Function:

CREATE OR REPLACE FUNCTION pictogram_frequency_on_delete()
RETURNS trigger AS
$BODY$
DECLARE

new_frequency RECORD;
target_unit RECORD;
current_row RECORD;
units_with_same_type RECORD;
what RECORD; 

BEGIN
SET search_path TO 'myScheme';

CASE TG_OP
WHEN 'DELETED' THEN what := OLD;
ELSE what:= OLD;
END CASE;

SELECT unit_type_uid INTO STRICT target_unit 
FROM unit 
WHERE unit_uid = what.unit_uid;

SELECT count(*) AS exists INTO STRICT current_row 
FROM unit_type_pictogram utp 
WHERE utp.pictogram_uid = what.pictogram_uid 
AND utp.unit_type_uid = target_unit.unit_type_uid;

IF (current_row.exists = 0) THEN
    RETURN what; /* return new/old doesnt work too */
END IF;

UPDATE unit_type_pictogram utp 
SET frequency = frequency - 1
WHERE utp.pictogram_uid = what.pictogram_uid 
AND utp.unit_type_uid = target_unit.unit_type_uid;

RETURN what; /* return new/old doesnt work too */

END
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

Trigger:

CREATE TRIGGER on_delete_frequency
BEFORE DELETE
ON unit_pictogram
FOR EACH ROW
  EXECUTE PROCEDURE pictogram_frequency_on_delete();