juju sync-watch throws stacktrace consistently when working with subordinate units

[lazypower]

charles@xps13:~/projects/work/charms/trusty/flannel$ juju sync-watch flannel/1
Watching ....
Updating flannel/1
Traceback (most recent call last):
File "/home/charles/.juju-plugins/juju-sync-watch", line 190, in
changes = watch(opts.unit, opts.charm_dir, opts.retry, opts.quiet)
File "/home/charles/.juju-plugins/juju-sync-watch", line 115, in watch
handler.dispatch_update()
File "/home/charles/.juju-plugins/juju-sync-watch", line 98, in dispatch_update
if retry and unit_in_error(unit_name):
File "/home/charles/.juju-plugins/juju-sync-watch", line 132, in unit_in_error
unit = env.status()['Services'][service_name]['Units'][unit_name]
TypeError: 'NoneType' object has no attribute 'getitem'
ERROR subprocess encountered error code 1