File system level 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 ("Log-shipping standby" as described in
http://www.postgresql.org/docs/9.1/static/warm-standby.html)

I take frequent backups of the standby server:

1) Stop standby server (fast shutdown).
2) Rsync to another fileserver
3) Start standby server.

I just tried to recover one of these backups which *failed* with the
following errors:

2014-02-17 14:27:28 CET LOG: incomplete startup packet
2014-02-17 14:27:28 CET LOG: database system was shut down in recovery
at 2013-12-25 18:00:03 CET
2014-02-17 14:27:28 CET LOG: could not open file
"pg_xlog/00000001000001E300000061" (log file 483, segment 97): No such
file or directory
2014-02-17 14:27:28 CET LOG: invalid primary checkpoint record
2014-02-17 14:27:28 CET LOG: could not open file
"pg_xlog/00000001000001E300000060" (log file 483, segment 96): No such
file or directory
2014-02-17 14:27:28 CET LOG: invalid secondary checkpoint record
2014-02-17 14:27:28 CET PANIC: could not locate a valid checkpoint record
2014-02-17 14:27:29 CET FATAL: the database system is starting up
2014-02-17 14:27:29 CET FATAL: the database system is starting up
2014-02-17 14:27:30 CET FATAL: the database system is starting up
2014-02-17 14:27:30 CET FATAL: the database system is starting up
2014-02-17 14:27:31 CET FATAL: the database system is starting up
2014-02-17 14:27:31 CET FATAL: the database system is starting up
2014-02-17 14:27:32 CET FATAL: the database system is starting up
2014-02-17 14:27:33 CET FATAL: the database system is starting up
2014-02-17 14:27:33 CET FATAL: the database system is starting up
2014-02-17 14:27:33 CET LOG: startup process (PID 26186) was terminated
by signal 6: Aborted
2014-02-17 14:27:33 CET LOG: aborting startup due to startup process
failure

So it seems the server is missing some WAL files which are not
in the backup? Or is it simply not possible to take a backup of a
standby server in recovery?

Best,
Juergen