pg_basebackup on standby node failed

hi, All,

I intend to do a basebackup on a hot standby node. I followed  the instructions on http://www.postgresql.org/docs/9.2/static/app-pgbasebackup.html to turn full_page_writes on at master and other settings on standby.

When I try to select pg_start_backup('backuplabel'); on the standby instance, it give me the following:
ERROR:  recovery is in progress
HINT:  WAL control functions cannot be executed during recovery.

I tried select  pg_xlog_replay_pause(); before the select pg_start_backup('backuplabel'); but that does not help.

Also why select pg_is_in_recovery(); always returns true when select  pg_is_xlog_replay_paused(); is true? I thought when I do select  pg_xlog_replay_pause(); the standby should not be in recovery

Please help if you know anything about this. Thank you.

best,
Ying