Reposting a comment from @bworrell since it got lost somehow:

> You may want to allow None being passed into these methods and allow them to return None. We do that in python-cybox and python-stix: [example](https://github.com/CybOXProject/python-cybox/blob/master/cybox/core/object.py#L144). Also, the method seems to assume the presence of keys, which may not be there (I think). Consider using the dictionary get() method which returns None by default if the key is not present, or you can set the default value.