unexpected PQresultStatus: 8 with simple logical replication

Hi all,

I'm trying to learn about how to subscribe to events in a logical
replication slot. I'm finding that the documentation is sparse and mostly
focused on binary WAL replication. I'd like to get a toy example working
where I can see changes in my terminal. I'm able to create a replication
slot, but trying to start it results in the error in the subject of my
email:

psql "dbname=postgres replication=database user=postgres" -c
"CREATE_REPLICATION_SLOT foobar LOGICAL test_decoding"
slot_name | consistent_point | snapshot_name | output_plugin
-----------+------------------+---------------+---------------
foobar | 0/D9C59A60 | 0000127E-1 | test_decoding
(1 row)

psql "dbname=postgres user=postgres" -c "SELECT slot_name, plugin,
restart_lsn FROM pg_replication_slots"

slot_name | plugin | restart_lsn
-----------+---------------+-------------
foobar | test_decoding | 0/D9C59A28
(1 row)

psql "dbname=postgres replication=database user=postgres" -c
"START_REPLICATION SLOT foobar LOGICAL 0/D9C59A28"
unexpected PQresultStatus: 8

Advice?
Many thanks,
Brent