nbsite build command fails with cryptic error

[bmorris3]

Running nbsite with the following command:

nbsite build --what=html --examples=../examples --doc=./ --output=_build/html

I get the following error:

Exception occurred:
  File "/Users/bmmorris/git/kelp/.tox/build_docs/lib/python3.7/site-packages/nbsite/nbbuild.py", line 56, in kc
    msg_id = self._kc.execute( # noqa: a mess
AttributeError: 'NoneType' object has no attribute 'execute'

Any tips? Thanks!