control reaches end of trigger procedure without RETURN

create function duplicate_images() returns OPAQUE as'
DECLARE
myrec RECORD;
BEGIN
if TG_OP = ''INSERT'' then
select * into myrec from images where page=NEW.page and
path=NEW.path;
if found then
raise exception ''insert failed: duplicate record found'';
end if;
end if;
end;
' language 'plpgsql';