9.6beta2: query failure with 'cache lookup failed for type 0'

Hello,
re-testing our application Openbravo on 9.6beta2 i found the following query failing to run with

ERROR: cache lookup failed for type 0

Tested on 9.6beta2
Specifically debian package version '9.6~beta2-1.pgdg+1' from apt.postgresql.org

SELECT
SUM(C_ORDERLINE.LINENETAMT),
C_CURRENCY_SYMBOL2 (SUM(C_ORDERLINE.LINENETAMT))

FROM C_ORDER, C_ORDERLINE
WHERE C_ORDER.C_ORDER_ID = C_ORDERLINE.C_ORDER_ID
GROUP BY C_ORDER.DOCUMENTNO
ORDER BY C_ORDER.DOCUMENTNO;

Note: query is slimmed down manually which still reproduces the issue (cut down from biggger query), which explain the kind of (now)(now) useless pl-function.

To reproduce in new empty database:
CREATE OR REPLACE FUNCTION public.c_currency_symbol2(p_amount numeric)
RETURNS character varying AS
$BODY$ DECLARE
BEGIN
RETURN p_amount;
END ; $BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

create table c_order (c_order_id varchar(32) primary key, documentno varchar(60));

create table c_orderline (c_orderline_id varchar(32) primary key, c_order_id varchar(32), linenetamt numeric, c_currency_id varchar(32));

Deleting any more out of the query seems to no longer trigger the problem.

Also changing the 'c_orderline' create table statement to not have the last column 'c_currency_id' (which is not even referenced in the query) also makes the issue no longer reproducible.

Regards,
Stefan