Backup of shut down standby does not work?

Hi all,

I have a master-slave configuration running the master with WAL
archiving enabled and the slave in recovery mode reading back the WAL
files from the master (as described in
http://www.postgresql.org/docs/9.1/static/continuous-archiving.html#BACKUP-PITR-RECOVERY

Now I am doing the following backup strategy:

1) Stop slave (fast shutdown).
2) Rsync to backup fileserver
3) Start slave.

I just tried to recover a Backup which *failed* with the following errors:

2014-02-13 08:34:05 CET LOG: starting archive recovery
2014-02-13 08:34:05 CET LOG: incomplete startup packet
cp: cannot stat `/var/postgres-wal/00000001000001E300000061': No such
file or directory
2014-02-13 08:34:05 CET LOG: could not open file
"pg_xlog/00000001000001E300000061" (log file 483, segment 97): No such
file or directory
2014-02-13 08:34:05 CET LOG: invalid primary checkpoint record
cp: cannot stat `/var/postgres-wal/00000001000001E300000060': No such
file or directory
2014-02-13 08:34:05 CET LOG: could not open file
"pg_xlog/00000001000001E300000060" (log file 483, segment 96): No such
file or directory
2014-02-13 08:34:05 CET LOG: invalid secondary checkpoint record
2014-02-13 08:34:05 CET PANIC: could not locate a valid checkpoint record
2014-02-13 08:34:06 CET FATAL: the database system is starting up
2014-02-13 08:34:06 CET FATAL: the database system is starting up
2014-02-13 08:34:07 CET FATAL: the database system is starting up
2014-02-13 08:34:07 CET FATAL: the database system is starting up
2014-02-13 08:34:07 CET LOG: startup process (PID 16882) was terminated
by signal 6: Aborted
2014-02-13 08:34:08 CET LOG: aborting startup due to startup process
failure

So it seems the server is missing the archived WAL files which are not
in the backup. Some time ago there was a discussion about this
(http://www.postgresql.org/message-id/CAFwQ8rd+oDkLG4KWkux=2jwNinBnuwhY8DH3W-_58MGO1m3=Lg@mail.gmail.com)
and it turned out that the WAL archive is not needed, only files in
pg_xlog which I *did* backup.

So am I doing something wrong or are the WAL files really needed?

Best,
Juergen