BUG #16374: I can't directly change owner from my created database to my created user.

The following bug has been logged on the website:

Bug reference: 16374
Logged by: wang yakun
Email address: 369358123@qq.com
PostgreSQL version: 10.10
Operating system: centos
Description:

[postgres10@pgcore-split-0001 ~]$ psql -Upostgres
psql (10.10)
Type "help" for help.

postgres=# create user root with login nosuperuser inherit createdb

createrole replication;

CREATE ROLE
postgres=# create role rdsgroup with nologin nosuperuser inherit

nocreatedb nocreaterole noreplication;

CREATE ROLE
postgres=# grant pg_monitor, pg_read_all_stats, pg_signal_backend,

pg_stat_scan_tables, rdsgroup to root;

GRANT ROLE
postgres=# \c postgres root
You are now connected to database "postgres" as user "root".
postgres=> CREATE DATABASE db1 WITH ENCODING = 'UTF8';
CREATE DATABASE
postgres=> create user user1 password '';
NOTICE: empty string is not a valid password, clearing password
CREATE ROLE
postgres=> alter user user1 with login;
ALTER ROLE
**postgres=> alter database db1 owner to user1;**
**2020-04-17 16:37:04.328 CST [4712] ERROR: must be member of role

"user1"**

**2020-04-17 16:37:04.328 CST [4712] STATEMENT: alter database db1 owner

to user1;**

**ERROR: must be member of role "user1"**

My user is "root".I know that before execute "alter database" stmt, I can
login with user1 and execute "grant user1 to db1", so this "alter database"
stmt will work.

However, "user1" is my created, and "db1" is my created too. Why couldn't I
chenge owner of "db1" to "user1" directly?