Traceback during upgrade from maas 2.1.3 to 2.2.0~rc1

I started with a maas cluster running 2.1.3+bzr5573-0ubuntu1~16.04.1. The attempt to upgrade to 2.2.0~rc1+bzr5922-0ubuntu1~16.04.1 from ppa:maas/next resulted in a Traceback while install maas-region-api. The maas cluster was configured with machines and was in use.

[from 'apt-get install maas']
...
Setting up maas-region-api (2.2.0~rc1+bzr5922-0ubuntu1~16.04.1) ...
Installing new version of config file /etc/maas/preseeds/curtin_userdata ...
Installing new version of config file /etc/maas/preseeds/enlist_userdata ...
Traceback (most recent call last):
  File "/usr/sbin/maas-region", line 65, in <module>
    main()
  File "/usr/sbin/maas-region", line 61, in main
    run()
  File "/usr/sbin/maas-region", line 49, in run
    management.execute_from_command_line()
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 354, in execute_from_command_line
    utility.execute()
  File "/usr/lib/python3/dist-packages/django/core/management/__init__.py", line 328, in execute
    django.setup()
  File "/usr/lib/python3/dist-packages/django/__init__.py", line 18, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/usr/lib/python3/dist-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File "/usr/lib/python3/dist-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "/usr/lib/python3/dist-packages/maasserver/models/__init__.py", line 99, in <module>
    from maasserver.models.bmc import (
  File "/usr/lib/python3/dist-packages/maasserver/models/bmc.py", line 44, in <module>
    from maasserver.models.iscsiblockdevice import (
  File "/usr/lib/python3/dist-packages/maasserver/models/iscsiblockdevice.py", line 10, in <module>
    from curtin.block.iscsi import IscsiDisk
ImportError: No module named 'curtin.block.iscsi'
dpkg: error processing package maas-region-api (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of maas-dns:
 maas-dns depends on maas-region-api (= 2.2.0~rc1+bzr5922-0ubuntu1~16.04.1); however:
  Package maas-region-api is not configured yet.

dpkg: error processing package maas-dns (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of maas-region-controller:
 maas-region-controller depends on maas-dns (= 2.2.0~rc1+bzr5922-0ubuntu1~16.04.1); however:
  Package maas-dns is not configured yet.
 maas-region-controller depends on maas-region-api (= 2.2.0~rc1+bzr5922-0ubuntu1~16.04.1); however:
  Package maas-region-api is not configured yet.

dpkg: error processing package maas-region-controller (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of maas:
 maas depends on maas-region-controller (= 2.2.0~rc1+bzr5922-0ubuntu1~16.04.1); however:
  Package maas-region-controller is not configured yet.

dpkg: error processing package maaNo apport report written because the error message indicates its a followup error
from a previous failure.
                        No apport report written because the error message indicates its a followup error from a pre
vious failure.
              No apport report written because MaxReports is reached already
                                                                            s (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 maas-region-api
 maas-dns
 maas-region-controller
 maas
E: Sub-process /usr/bin/dpkg returned an error code (1)