sql function with empty row

Hello,

I have defined a SQL function

CREATE OR REPLACE FUNCTION substancetrivialname(text)
RETURNS substance
LANGUAGE 'sql'
COST 100
VOLATILE
AS $BODY$
select s.* from substancetrivialname n
join substance s on s.id = n.idsubstance
where lower(btrim(n.name)) = lower(btrim($1));
$BODY$;

substance and substancetrivialname have got a 1-to-N relationship (for each substance can exist multiple trivial names).
If I call the function with a non-existing trivial name it returns a single row with all fields are set to NULL.

If I run the join query directly it returns an empty record set on a non-existing trivial name.
I expected equal behavior on my function, so my question is, how can I fix this?

Thanks

Phil