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(at)arkia(dot)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;