update item fails with 500 error for non-existent item

According to API doc:

update_item
If no item with the specified key is found:
PUT - MagnetoDB creates a new item with the specified primary key, and then adds the attribute.
DELETE - Nothing happens; there is no attribute to delete.
ADD - MagnetoDB creates an item with the supplied primary key and number (or set of numbers) for the attribute value. The only data types allowed are number and number set; no other data types can be specified.

Steps to reproduce:

1. create a table with 3 fields, one field as hash key, one field as range key
2. update_item, with attribute_updates set to a new attribute of type SS, such as
    "attribute_updates": {
        "Tags": {
            "action": "PUT",
          "value": {
            "SS": ["some string"]
          }
        }
3. API returns the following error response:
explanation: "The server has either erred or is incapable of performing the requested operation."
code: 500
-error: {
message: "'NoneType' object has no attribute 'iteritems'"
traceback: null
type: "AttributeError"
}
title: "Internal Server Error"

MagnetoDB log:

2014-09-24 17:21:29.266 15395 ERROR magnetodb.common.middleware.fault [-] 'NoneType' object has no attribute 'iteritems'
Traceback (most recent call last):
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/middleware/fault.py", line 118, in process_request
    return req.get_response(self.application)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/request.py", line 1320, in send
    application, catch_exc_info=False)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/request.py", line 1284, in call_application
    app_iter = application(self.environ, start_response)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/dec.py", line 130, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/dec.py", line 195, in call_func
    return self.func(req, *args, **kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/wsgi.py", line 66, in __call__
    response = req.get_response(self.application)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/request.py", line 1320, in send
    application, catch_exc_info=False)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/request.py", line 1284, in call_application
    app_iter = application(self.environ, start_response)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/dec.py", line 144, in __call__
    return resp(environ, start_response)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/routes/middleware.py", line 131, in __call__
    response = self.app(environ, start_response)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/dec.py", line 144, in __call__
    return resp(environ, start_response)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/dec.py", line 130, in __call__
    resp = self.call_func(req, *args, **self.kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/.venv/lib/python2.7/site-packages/webob/dec.py", line 195, in call_func
    return self.func(req, *args, **kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/wsgi.py", line 250, in __call__
    action_result = self.execute_action(action, request, **action_args)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/wsgi.py", line 264, in execute_action
    return self.dispatch(self.controller, action, request, **action_args)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/wsgi.py", line 273, in dispatch
    return method(*args, **kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/probe.py", line 94, in wrapper
    return f(*args, **kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/api/openstack/v1/update_item.py", line 108, in process_request
    expected_condition_map=expected_item_conditions)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/storage/__init__.py", line 295, in update_item
    expected_condition_map
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/storage/manager/simple_impl.py", line 523, in update_item
    expected_condition_map
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/common/probe.py", line 94, in wrapper
    return f(*args, **kwargs)
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/storage/driver/cassandra/cassandra_impl.py", line 1144, in update_item
    key_attribute_map, old_item
  File "/Users/charles_wang1/dev/work/magnetodb/magnetodb/storage/driver/cassandra/cassandra_impl.py", line 1273, in _get_update_conditions
    for attr_name, attr_value in old_item.iteritems():
AttributeError: 'NoneType' object has no attribute 'iteritems'

127.0.0.1 - - [24/Sep/2014 17:21:29] "POST /v1/default_tenant/data/tables/Thread/update_item HTTP/1.1" 500 417 0.009473