repmgr problem with registering standby

Hi,
I have repmgr working to some degree on a couple of servers, but am
having trouble with the "register" part on the slave.

On the master, I run:
# repmgr -f /etc/repmgr/validator/repmgr.conf \
--verbose --force master register

Opening configuration file: /etc/repmgr/validator/repmgr.conf
repmgr connecting to master database
repmgr connected to master, checking its state
finding node list for cluster 'validator'
Master node correctly registered for cluster validator with id 0
(conninfo: host=10.133.54.2 port=5432 user=repmgr dbname=repmgr)

So that looks good, but then I try this on the slave:
# repmgr -f /etc/repmgr/validator/repmgr.conf \
--verbose standby register

Opening configuration file: /etc/repmgr/validator/repmgr.conf
repmgr connecting to standby database
repmgr connected to standby, checking its state
repmgr connecting to master database
finding node list for cluster 'validator'
A master must be defined before configuring a slave

I can query the database like so though, and it seems like it's all good:
repmgr=# select * from repmgr_validator.repl_nodes;
id | cluster | conninfo
----+-----------+------------------------------------------------------
0 | validator | host=10.133.54.2 port=5432 user=repmgr dbname=repmgr
(1 row)

Does anyone have an idea of what might be going wrong here?

Thanks,
Toby