Python exception trying to display an image with no tags

[cmihai]

Traceback (most recent call last):
  File "/usr/bin/docktree", line 11, in <module>
    sys.exit(main())
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 115, in main
    Docktree(**vars(args)).draw_tree()
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 103, in draw_tree
    draw_branch(prune(tree()))
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 79, in tree
    return [(Image(img_dict, self.show_size), tree(img_dict['Id'])) for img_dict in children(root_id)]
  File "/usr/lib/python2.7/site-packages/docktree/__init__.py", line 45, in __init__
    self.tags = [t for t in img_dict['RepoTags'] if t != '<none>:<none>']
TypeError: 'NoneType' object is not iterable