On 2019-03-08 17:38:33 +0000, PG Bug reporting form wrote:

 The following bug has been logged on the website:

 Bug reference: 15678
 Logged by: Vasiliy Korotkikh
 Email address: dhaenoor@yandex.ru
 PostgreSQL version: 11.2
 Operating system: Debian Stretch
 Description:

 In the configuration file of my server there is the following line:
 default_transaction_isolation = 'serializable'

Did you reload the configuration after making that change? SELECT
pg_reload_conf();

Greetings,

Andres Freund