simplekv: simplekv 0.11.1 breaks flask-kvsession 0.6.2 on Python 2.7

simplekv 0.11.1 breaks flask-kvsession 0.6.2 sid_s created by kvsession is of type str rather than the expected type unicode

Traceback (most recent call last):
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/flask/app.py", line 1479, in full_dispatch_request
    response = self.process_response(response)
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/flask/app.py", line 1693, in process_response
    self.save_session(ctx.session, response)
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/flask/app.py", line 837, in save_session
    return self.session_interface.save_session(self, session, response)
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/flask_kvsession/__init__.py", line 193, in save_session
    store.put(session.sid_s, data, ttl)
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/simplekv/decorator.py", line 73, in put
    self._dstore.put(self._map_key(key), *args, **kwargs))
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/simplekv/decorator.py", line 104, in _map_key
    self._check_valid_key(key)
  File "/opt/my-flask-app/ENV/local/lib/python2.7/site-packages/simplekv/__init__.py", line 190, in _check_valid_key
    raise ValueError('%r is not a unicode string' % key)