'mariabackup --prepare': Cannot config file, backup-my.cnf

I am testing mariabackup. I could successfully make a mariadb backup using mariabackup utility. As the manual said before restore (-copy-back), we should use '-prepare' option first.

Therefore, I executed the following statement:

1. mariabackup --defaults-file=/etc/my.cnf.d/backup-my.cnf --prepare --target-dir=/backup/mariadb-backup-20181210182451

mariabackup based on MariaDB server 10.3.10-MariaDB Linux (x86_64)
mariabackup: cd to /backup/mariadb-backup-20181210182451/
cannot open backup-my.cnf for reading

It complained 'cannot open backup-my.cnf for reading'.

Then I tried the following:

1. mariabackup --defaults-file="/etc/my.cnf.d/backup-my.cnf" --prepare --target-dir=/backup/mariadb-backup-20181210182451

Same error messages!

I have already created 'backup-my.cnf' under /etc/my.cnf.d

cd /etc/my.cnf.d

[root@db1 my.cnf.d]# ll
total 16
rw-rr-. 1 root root 739 Dec 10 18:42 backup-my.cnf
rw-rr-. 1 root root 763 Oct 2 17:45 enable_encryption.preset
rw-rr-. 1 root root 232 Oct 2 17:45 mysql-clients.cnf
rw-rr-. 1 root root 1789 Dec 10 18:40 server.cnf

Why did mariabackup complain 'cannot open backup-my.cnf for reading'?