Description of problem:
Tried to run playbook which contained badly defined tags:
   tags:                                                                                                                                                                                                                         
       - [ 'prepare-env', 'lxc_container' ]

Version-Release number of selected component:
ansible-1.7.2-1.fc20

Additional info:
reporter:       libreport-2.2.3
cmdline:        /usr/bin/python /usr/bin/ansible-playbook home-dc.yml --limit net-warez --tags lxc_container --list-tasks
executable:     /usr/bin/ansible-playbook
kernel:         3.16.4-200.fc20.x86_64
runlevel:       N 5
type:           Python
uid:            1000

Truncated backtrace:
play.py:750:_late_merge_role_tags:TypeError: unhashable type: 'list'

Traceback (most recent call last):
  File "/usr/bin/ansible-playbook", line 309, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/usr/bin/ansible-playbook", line 202, in main
    vault_password=pb.vault_password)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/play.py", line 161, in __init__
    self._late_merge_role_tags()
  File "/usr/lib/python2.7/site-packages/ansible/playbook/play.py", line 750, in _late_merge_role_tags
    self._tasks[idx].tags = sorted(set(self._tasks[idx].tags + role_tags[this_role]))
TypeError: unhashable type: 'list'

Local variables in innermost frame:
task: {'meta': 'flush_handlers'}
val: <ansible.playbook.task.Task object at 0x1657140>
idx: 1
self: <ansible.playbook.play.Play object at 0x1646880>
role_tags: {'prepare-env-78a7eae7-d59f-469c-b7f4-2561805e571e': []}
this_role: 'prepare-env-78a7eae7-d59f-469c-b7f4-2561805e571e'