machine allocation on a (virsh) pod fails: frozenset does not support indexing

Having converted my machine to use virsh pod, I get the following error when juju (2.2~alpha1) tries to allocate a machine:

2017-03-21 12:26:21 maasserver: [error] ################################ Exception: 'frozenset' object does not support indexing ################################
2017-03-21 12:26:21 maasserver: [error] Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 177, in view_atomic_with_post_commit_savepoint
    return view_atomic(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/maasserver/api/support.py", line 59, in __call__
    response = upcall(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/decorators/vary.py", line 21, in inner_func
    response = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/piston3/resource.py", line 190, in __call__
    result = self.error_handler(e, request, meth, em_format)
  File "/usr/lib/python3/dist-packages/piston3/resource.py", line 188, in __call__
    result = meth(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/api/support.py", line 298, in dispatch
    return function(self, request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/api/machines.py", line 1339, in allocate
    architecture = architectures[0]
TypeError: 'frozenset' object does not support indexing