Keystone fails when returning unscoped federated token as XML

Keystone fails when unscoped token is serialized to the XML output. 'OS-FEDERATION:groups' tag is invalid.

Traceback:

ERROR keystone.middleware.core Serializer failed
TRACE keystone.middleware.core Traceback (most recent call last):
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/middleware/core.py", line 167, in process_response
TRACE keystone.middleware.core response.body = serializer.to_xml(body_obj)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 67, in to_xml
TRACE keystone.middleware.core return serialize(d, xmlns)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 218, in __call__
TRACE keystone.middleware.core self.populate_element(root, d[name])
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 323, in populate_element
TRACE keystone.middleware.core self._populate_tree(element, value)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 357, in _populate_tree
TRACE keystone.middleware.core self._populate_dict(element, k, v)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 294, in _populate_dict
TRACE keystone.middleware.core self.populate_element(child, v)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 323, in populate_element
TRACE keystone.middleware.core self._populate_tree(element, value)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 359, in _populate_tree
TRACE keystone.middleware.core self._populate_list(element, k, v)
TRACE keystone.middleware.core File "/opt/stack/keystone/keystone/common/serializer.py", line 272, in _populate_list
TRACE keystone.middleware.core container = etree.Element(k)
TRACE keystone.middleware.core File "lxml.etree.pyx", line 2811, in lxml.etree.Element (src/lxml/lxml.etree.c:65352)
TRACE keystone.middleware.core File "apihelpers.pxi", line 103, in lxml.etree._makeElement (src/lxml/lxml.etree.c:13888)
TRACE keystone.middleware.core File "apihelpers.pxi", line 1575, in lxml.etree._tagValidOrRaise (src/lxml/lxml.etree.c:27942)
TRACE keystone.middleware.core ValueError: Invalid tag name u'OS-FEDERATION:groups'