Sudden FTS-related error from parallel worker in 9.6

I have a Postgres 9.6.0 instance for testing purposes. It turned out that
in postgresql.conf, this postgres cluster has "default_text_search_config"
being set to "ru" for some reason.

Suddenly, I encountered with an error relating to full text configuration
in test database, where I didn't use FTS at all, I just tried to test
parallel aggregation feature:

test=# create table test as
test-# select
test-# i as id,
test-# now() - random() * '5 years'::interval as ts,
test-# (random() * 100000000)::int4 as number,
test-# repeat('x', (10 + 40 * random())::int4) as txt
test-# from
test-# generate_series(1, 10000000) i;
SELECT 10000000

test=# alter table test add primary key (id);
ALTER TABLE

test=# explain analyze select min(ts) from test;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=226505.45..226505.46 rows=1 width=8) (actual
time=1460.227..1460.227 rows=1 loops=1)
-> Seq Scan on test (cost=0.00..201504.76 rows=10000276 width=8)
(actual time=0.013..697.676 rows=10000000 loops=1)
Planning time: 0.098 ms
Execution time: 1460.257 ms
(4 rows)

test=# set max_parallel_workers_per_gather to 8;
SET
test=# explain analyze select min(ts) from test;
*ERROR: invalid value for parameter "default_text_search_config": "ru"*
*CONTEXT: parallel worker*

Such an error would be OK if I tried to use some text search function
(since really no 'ru' configuration is present in 'test' database), but I
ovbviously didn't.

As you can see, no FTS features are involved in table definition, so it
seems really strange to see FTS-related error

Bottom line, steps to reproduce:
1) put not existing text search default configurtion
2) set max_parallel_workers_per_gather to 8;
3) do some aggregation query that would involved parallel workers and
doesn't use FTS features.

What I expected: no FTS-related error, because I FTS was not used by the
query.