Slave relation fails: IndexError: list index out of range

Discourse bug reported with reproduction steps: https://discourse.juju.is/t/help-with-percona-cluster-to-cluster-replication/3132

Steps to reproduce:

juju deploy percona-cluster master --series bionic --constraints "mem=2048"
juju config master vip=$TEST_VIP00
juju config master cluster-id=1
juju config master min-cluster-size=3
juju deploy hacluster master-hacluster
juju relate master-hacluster master
juju add-unit master -n 2

juju deploy keystone
juju add-relation keystone master

juju deploy percona-cluster slave --series bionic --constraints "mem=2048"
juju config slave vip=$TEST_VIP01
juju config slave cluster-id=2
juju config slave min-cluster-size=3
juju deploy hacluster slave-hacluster
juju relate slave-hacluster slave
juju add-unit slave -n 2

# Wait for the model to settle to confirm both clusters are healthy

juju config master databases-to-replicate=keystone
juju config slave databases-to-replicate=keystone
juju relate master:master slave:slave

Confirmed
2020-05-28 23:11:48 DEBUG slave-relation-changed Traceback (most recent call last):
2020-05-28 23:11:48 DEBUG slave-relation-changed File "/var/lib/juju/agents/unit-slave-1/charm/hooks/slave-relation-changed", line 1148, in <module>
2020-05-28 23:11:48 DEBUG slave-relation-changed main()
2020-05-28 23:11:48 DEBUG slave-relation-changed File "/var/lib/juju/agents/unit-slave-1/charm/hooks/slave-relation-changed", line 1138, in main
2020-05-28 23:11:48 DEBUG slave-relation-changed hooks.execute(sys.argv)
2020-05-28 23:11:48 DEBUG slave-relation-changed File "/var/lib/juju/agents/unit-slave-1/charm/charmhelpers/core/hookenv.py", line 943, in execute
2020-05-28 23:11:48 DEBUG slave-relation-changed self._hooks[hook_name]()
2020-05-28 23:11:48 DEBUG slave-relation-changed File "/var/lib/juju/agents/unit-slave-1/charm/hooks/slave-relation-changed", line 1114, in slave_changed
2020-05-28 23:11:48 DEBUG slave-relation-changed if rdata.get('master_address') is not get_slave_status():
2020-05-28 23:11:48 DEBUG slave-relation-changed File "/var/lib/juju/agents/unit-slave-1/charm/hooks/percona_utils.py", line 1422, in get_slave_status
2020-05-28 23:11:48 DEBUG slave-relation-changed return results[0][1]
2020-05-28 23:11:48 DEBUG slave-relation-changed IndexError: list index out of range