Passed invalid arguments: argument of type 'NoneType' is not iterable.
Usage:
    Return top-level images (those on which no other images depend) which do
    not have a tag assigned to them. These include:
    - Images which were once tagged but were later untagged, such as those
      which were superseded by committing a new copy of an existing tagged
      image.
    - Images which were loaded using :py:func:`docker.load
      <salt.modules.dockerng.load>` (or the ``docker load`` Docker CLI
      command), but not tagged.
    prune : False
        Remove these images
    force : False
        If ``True``, and if ``prune=True``, then forcibly remove these images.
    **RETURN DATA**
    If ``prune=False``, the return data will be a list of dangling image IDs.
    If ``prune=True``, the return data will be a dictionary with each key being
    the ID of the dangling image, and the following information for each image:
    - ``Comment`` - Any error encountered when trying to prune a dangling image
      *(Only present if prune failed)*
    - ``Removed`` - A boolean (``True`` if prune was successful, ``False`` if
      not)
    CLI Example:
    .. code-block:: bash
        salt myminion dockerng.dangling
        salt myminion dockerng.dangling prune=True

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/cli/caller.py", line 197, in call
    ret['return'] = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/salt/modules/dockerng.py", line 3610, in dangling
    if '<none>:<none>' in all_images[x]['RepoTags']]
TypeError: argument of type 'NoneType' is not iterable