Nipype MapNode and JSONFileGrabber

Dear Nipype gurus, I cannot seem to get MapNode and JSONFileGrabber to cooperate. Here is my problem. I’ve got a couple of JSON filenames in a list:

filenames = ['/path/to/a.json', '/path/to/b.json']
json_mapnode = MapNode(JSONFileGrabber(), iterfield=['in_file'], name='x')
json_mapnode.inputs.in_file = filenames
r =

I get an empty Bunch(). :frowning:
I suspect a silly mistake or something unique about JSONFileGrabber as my other MapNodes work as expected and JSONFileGrabber also works as expected until I wrap it in a MapNode.

Any hints are greatly appreciated!