See comment #1155 (comment)

When serializing error data (traceback, local vars, etc.) to be sent to the backend, json.dumps is used, in raven.utils.json. json.dumps can fail, e.g. when local data has the form:

{ (): "foo" }

which cannot be serialized to json (because of the tuple key).
By using json.dumps(skipkeys=True) these kind of keys will be ignored.