BUG #14017: strange syntax error

The following bug has been logged on the website:

Bug reference:      14017
Logged by:          Anthony van den Berg
Email address:      anthony@arkia.nl
PostgreSQL version: 9.5.1
Operating system:   linux both fedra and ubuntu
Description:

When running the create or replace function on the following I get this
error

ERROR:  syntax error at or near "("
LINE 5: ...trim(isnull(new.straat,''::text) || ' ' || isnull (  new.hui...


in postgresql 9.4.5 it gives no problem

I have no idea why this happens ans the first isnull call is exactly the
same als the second

CREATE OR REPLACE FUNCTION relaties.update_insert_adressen()
  RETURNS trigger AS
$BODY$
BEGIN
    new.adres = trim(isnull(new.straat,''::text) || ' ' || isnull (
new.huisnummer,''::text));
    return new;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION relaties.update_insert_adressen()
  OWNER TO postgres;



the isnull function looks like this.



CREATE OR REPLACE FUNCTION public."isnull"(
    text,
    text)
  RETURNS text AS
$BODY$SELECT (CASE (SELECT $1
is null) WHEN true THEN $2 ELSE $1 END) AS RESULT$BODY$
  LANGUAGE sql VOLATILE
  COST 100;
ALTER FUNCTION public."isnull"(text, text)
  OWNER TO postgres;