default_login_user setting is not honored in 2.4.x

i have in my dovecot.conf:

default_login_user = www

when i start dovecot:

Starting mail server ...doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf: default_login_user doesn't exist: dovenull

after adding dovenull user to /etc/passwd _AND_ leaving "default_login_user = www" in the dovecot.conf file everything is working fine. With 2.3.x user www worked.