Replication Issues

Hi Team,

I have configured replication using slot ,But it failed by throwing the
ERROR *pg_basebackup: could not get transaction log end position from
server: ERROR: requested WAL segment 0000000100002C9D00000085 has already
been removed *, which is unexpected because i have created the slot on
master first and then issued the base backup command from slave's end
the command is
*usr/lib/postgresql/9.5/bin/pg_basebackup -U user --max-rate='150 M'
--progress --verbose --write-recovery-conf --status-interval='10 s' -D
data_dir -h host_ip -p 5433*
These are the settings on my master
archive_mode=on
archive_command='/bin/true'
wal_keep_segments=512
max_wal_senders=4
Series of steps i have followed :
1) Enabled password less authentication between master and slave
2)created slot on master (assuming it will store wal's regardless of other
settings)
3)started basebackup from slave's end
4)Issued checkpoint at master's end

Here my concern is , slave should recover WAL from replication slot but why
i got the above ERROR , Why slot removed the requested wal file , Could
you please let me know the reason why it happened or did i miss something ?

Thanks,
Bhargav K