postgresql replay_location in pg_stat_replication is empty

I'm running a cluster of PostgreSQL 9.5.2, and export the stream log with confluentinc/bottledwater-pg: Change data capture from PostgreSQL into Kafka. The replication works fine, but I'm wondering about the output of pg_stat_replication:

db# select * from pg_stat_replication ;
-[ RECORD 1 ]----+------------------------------
pid              | 371125
usesysid         | 16385
usename          | db
application_name | bottledwater
client_addr      | 127.0.0.1
client_hostname  |
client_port      | 22338
backend_start    | 2016-09-30 16:58:04.074937+08
backend_xmin     |
state            | streaming
sent_location    | 12DD/D325D478
write_location   | 12DD/D2D75EF0
flush_location   | 12DD/D2B8B5F8
replay_location  |
sync_priority    | 0
sync_state       | async

See near the bottom, the replay_location is null. If anyone knows what's wrong, let me know.