innobackupex don't take the default datadir

If you don't have any "datadir" option on you mysqld section of your my.cnf, the server start fine and use /var/lib/mysql as a default but innobackupex don't

[root@repmaster ~]# innobackupex backups/

InnoDB Backup Utility v1.5.1-xtrabackup; Copyright 2003, 2009 Innobase Oy
and Percona Inc 2009-2012. All Rights Reserved.

This software is published under
the GNU GENERAL PUBLIC LICENSE Version 2, June 1991.

120220 09:58:49 innobackupex: Starting mysql with options: --unbuffered --
120220 09:58:49 innobackupex: Connected to database with mysql child process (pid=2448)
120220 09:58:55 innobackupex: Connection to database server closed
IMPORTANT: Please check that the backup run completes successfully.
           At the end of a successful backup run innobackupex
           prints "completed OK!".

innobackupex: Using mysql Ver 14.14 Distrib 5.5.20, for Linux (x86_64) using readline 5.1
innobackupex: Using mysql server version Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

xtrabackup: Error: Please set parameter 'datadir'
innobackupex: fatal error: no 'mysqld' group in MySQL options
innobackupex: fatal error: OR no 'datadir' option in group 'mysqld' in MySQL options

[root@repmaster ~]# cat /etc/my.cnf | grep -v ^#

[client]
port = 3306
socket = /var/lib/mysql/mysql.sock

[mysqld]
port = 3306
socket = /var/lib/mysql/mysql.sock
server-id = 1
log-bin=mysql-bin

[mysqldump]
quick
max_allowed_packet = 16M

[mysql]
no-auto-rehash

[myisamchk]
key_buffer_size = 8M
sort_buffer_size = 8M

[mysqlhotcopy]
interactive-timeout

[root@repmaster ~]# rpm -qa | grep xtra
xtrabackup-1.6.5-328.rhel6.x86_64

[root@repmaster ~]# cat /etc/issue
CentOS release 6.2 (Final)

[root@repmaster ~]# rpm -qa | grep -i percona
Percona-Server-server-55-5.5.20-rel24.1.217.rhel6.x86_64
Percona-Server-devel-55-5.5.20-rel24.1.217.rhel6.x86_64
percona-release-0.0-1.x86_64
Percona-Server-shared-55-5.5.20-rel24.1.217.rhel6.x86_64
Percona-Server-client-55-5.5.20-rel24.1.217.rhel6.x86_64