xb 2.2 single binary fails against ps 5.1and 5.5 if innodb_log_file_size is not specified in server my.cnf

XtraBackup 2.2 (revno 4909) single binary will fail to backup Percona Server 5.1 (revno 600) and Percona Server 5.5 (revno 599) if the server does not have innodb_log_file_size explicitly set in it my.cnf. It works fine against Percona Server 5.6 (revno 508).

innobackupex stderr:

InnoDB Backup Utility v1.5.1-xtrabackup; Copyright 2003, 2009 Innobase Oy
and Percona LLC and/or its affiliates 2009-2013. All Rights Reserved.

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

131203 19:14:44 innobackupex: Connecting to MySQL server with DSN 'dbi:mysql:;mysql_read_default_file=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/my.cnf;mysql_read_default_group=xtrabackup' (using password: NO).
131203 19:14:44 innobackupex: Connected to MySQL server
131203 19:14:44 innobackupex: Executing a version check against the server...
131203 19:14:44 innobackupex: Done.
IMPORTANT: Please check that the backup run completes successfully.
           At the end of a successful backup run innobackupex
           prints "completed OK!".

innobackupex: Using mysql server version 5.1.72

innobackupex: Created backup directory /mnt/xb-2.2-single-binary/test-backup

131203 19:14:44 innobackupex: Starting ibbackup with command: xtrabackup --defaults-file="/mnt/xb-2.2-single-binary/var/test-0/mysql.1/my.cnf" --defaults-group="mysqld" --backup --suspend-at-end --target-dir=/mnt/xb-2.2-single-binary/test-backup --tmpdir=/tmp --extra-lsndir='/tmp'
innobackupex: Waiting for ibbackup (pid=11150) to suspend
innobackupex: Suspend file '/mnt/xb-2.2-single-binary/test-backup/xtrabackup_suspended_2'

xtrabackup version 2.2.0 based on MySQL server 5.6.14 Linux (x86_64) (revision id: )
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /mnt/xb-2.2-single-binary/var/test-0/mysql.1/data
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = ./
xtrabackup: innodb_data_file_path = ibdata1:10M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 50331648
InnoDB: Error: log file ./ib_logfile0 is of different size 5242880 bytes
InnoDB: than specified in the .cnf file 50331648 bytes!
innobackupex: Error: The xtrabackup child process has died at /mnt/xb-2.2-single-binary/xb-2.2-bin/innobackupex line 2663.

my.cnf:
[client]
port=10000
socket=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/mysql.sock
[mysqld]
basedir=/mnt/xb-2.2-single-binary/ps-5.1-bin
datadir=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/data
tmpdir=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/tmp
port=10000
socket=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/mysql.sock
pid-file=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/mysql.pid
console
user=root
server-id=1
general-log-file=/mnt/xb-2.2-single-binary/var/test-0/mysql.1/query.log
innodb_buffer_pool_size=512M

Explicitly adding innodb_log_file_size=512M to the my.cnf, deleting and recreating the database via mysql_install_db and re-running the same backup succeeds.