pg_last_xlog_receive_location()

In my understanding pg_last_xlog_receive_location() returns NULL if
applied to non standby server:

Get last transaction log location received and synced to disk by
streaming replication. While streaming replication is in progress this
will increase monotonically. But when streaming replication is
restarted this will back off to the replication starting position,
typically the beginning of the WAL file containing the current replay
location. If recovery has completed this will remain static at the
value of the last WAL record received and synced to disk during
recovery. If streaming replication is disabled, or if it has not yet
started, the function returns NULL.

However my primary server returns non NULL. Am I missing something?
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp