Req. for some help with resolving the error "pg_basebackup: could not get WAL end position from server: FATAL: requested WAL segment"

Hi

I am a newbie to postgres. We are on postgres 9.1 running on RedHat 6.  When we do the postgres backups we are noticing the following errors

pg_basebackup: could not get WAL end position from server: FATAL:  requested WAL segment  000000010000020B00000040 has already been removed

But when I look into pg_xlog, I notice

000000010000020B00000040.00000118.backup

May I know how to resolve this error? Are the backups done with these errors, recoverable?

Thanks in advance for your support.

Thanx,

Sam