Error 'unsupported operand type(s)' after 'rally task list'

(.venv)developer@fuel:stack$ rally task list
2015-04-13 10:45:23.472 16602 RALLYDEBUG rally.cmd.cliutils [-] INFO logs from urllib3 and requests module are hide.
2015-04-13 10:45:23.473 16602 RALLYDEBUG rally.cmd.cliutils [-] ERROR log from boto module is hide.
2015-04-13 10:45:23.473 16602 INFO rally.common.utils [-] Loading plugins from directories /opt/rally/plugins//*
2015-04-13 10:45:23.474 16602 INFO rally.common.utils [-] Loaded module with plugins: /opt/rally/plugins/keystone_plugin.py
2015-04-13 10:45:23.475 16602 INFO rally.common.utils [-] Loaded module with plugins: /opt/rally/plugins/sample_plugin.py
2015-04-13 10:45:23.476 16602 INFO rally.common.utils [-] Loaded module with plugins: /opt/rally/plugins/mox.py
2015-04-13 10:45:26.329 16602 ERROR rally.cmd.cliutils [-] unsupported operand type(s) for -: 'NoneType' and 'datetime.datetime'
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils Traceback (most recent call last):
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils File "/opt/stack/.venv/lib/python2.7/site-packages/rally/cmd/cliutils.py", line 452, in run
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils ret = fn(*fn_args, **fn_kwargs)
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils File "<string>", line 2, in list
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils File "/opt/stack/.venv/lib/python2.7/site-packages/rally/cmd/envutils.py", line 66, in default_from_global
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils return f(*args, **kwargs)
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils File "/opt/stack/.venv/lib/python2.7/site-packages/rally/cmd/commands/task.py", line 481, in list
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils x["duration"] = x["updated_at"] - x["created_at"]
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils TypeError: unsupported operand type(s) for -: 'NoneType' and 'datetime.datetime'
2015-04-13 10:45:26.329 16602 TRACE rally.cmd.cliutils
unsupported operand type(s) for -: 'NoneType' and 'datetime.datetime'