restore networking configuration results in error "Expecting value: line 1 column 1 (char 0)"

I'm getting an error using running "maas root machine restore-networking-configuration dqyy8h"

2020-02-10 18:40:30 regiond: [info] 10.244.40.31 POST /MAAS/api/2.0/machines/4mxs7d/?op=restore_networking_configuration HTTP/1.1 --> 500 INTERNAL_SERVER_ERROR (referrer: -; agent: Python-httplib2/0.9.2 (gzip))
2020-02-10 18:40:48 maasserver: [error] ################################ Exception: Expecting value: line 1 column 1 (char 0) ################################
2020-02-10 18:40:48 maasserver: [error] Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 291, in view_atomic_with_post_commit_savepoint
    return view_atomic(*args, **kwargs)
  File "/usr/lib/python3.6/contextlib.py", line 52, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/maasserver/api/support.py", line 57, 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 313, in dispatch
    return function(self, request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/api/machines.py", line 1294, in restore_networking_configuration
    machine.restore_network_interfaces()
  File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 4537, in restore_network_interfaces
    self, json.loads(script.output), NUMANode.objects.filter(node=self)
  File "/usr/lib/python3.6/json/__init__.py", line 354, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.6/json/decoder.py", line 357, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

logs: http://people.canonical.com/~jhobbs/logs-2020-02-10-18.48.52.tar

This is with maas 2.7.0~rc6-8229-g.fa71c492d-0ubuntu1~18.04.1