pg_archivecleanup not deleting anything?

Hello,

I'm running Postgres 9.3 in a warm standby configuration, and the slave
has this setting in recovery.conf:

archive_cleanup_command = '/usr/lib/postgresql/9.3/bin/pg_archivecleanup
/secure/pgsql/archive/ %r'

But I noticed that the archive directory had files going back to
February 2014:

$ ls -ltr archive | head
total 9126292
-rw------- 1 postgres postgres      300 Feb 15  2014
000000010000000000000002.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
000000010000000000000003.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
000000010000000000000004.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
000000010000000000000006.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
000000010000000000000007.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
00000001000000000000000B.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
00000001000000000000000C.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
00000001000000000000000E.00000028.backup
-rw------- 1 postgres postgres      300 Feb 15  2014
00000001000000000000000F.00000028.backup

And even if I run the command by hand, nothing is actually deleted:

$ pg_archivecleanup -d /secure/pgsql/archive/
00000001000000000000000F.00000028.backup
pg_archivecleanup: keep WAL file
"/secure/pgsql/archive//00000001000000000000000F" and later

Despite the message, and the same files remain.

Does anyone have any idea why pg_archivecleanup isn't deleting anything?

Thanks,
Paul