ansible 2.1.2.0
  config file = /home/user/a-bug-project/ansible.cfg
  configured module search path = Default w/o overrides

# site.yml

---

- hosts: all
  tasks:
    - include: test.yml

# test.yml

---
hosts: all

please note the missing "-" before hosts

cmd from dir where the files are located:
ansible-playbook site.yml

ansible-playbook site.yml -vvvv
Using /home/user/a-bug-project/ansible.cfg as config file
 [WARNING]: provided hosts list is empty, only localhost is available

ERROR! Unexpected Exception: global name 'AnsibleError' is not defined
the full traceback was:

Traceback (most recent call last):
  File "/usr/bin/ansible-playbook", line 92, in <module>
    exit_code = cli.run()
  File "/usr/lib/python2.7/site-packages/ansible/cli/playbook.py", line 154, in run
    results = pbex.run()
  File "/usr/lib/python2.7/site-packages/ansible/executor/playbook_executor.py", line 73, in run
    pb = Playbook.load(playbook_path, variable_manager=self._variable_manager, loader=self._loader)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/__init__.py", line 53, in load
    pb._load_playbook_data(file_name=file_name, variable_manager=variable_manager)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/__init__.py", line 98, in _load_playbook_data
    entry_obj = Play.load(entry, variable_manager=variable_manager, loader=self._loader)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/play.py", line 115, in load
    return p.load_data(data, variable_manager=variable_manager, loader=loader)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/base.py", line 219, in load_data
    self._attributes[name] = method(name, ds[name])
  File "/usr/lib/python2.7/site-packages/ansible/playbook/play.py", line 145, in _load_tasks
    return load_list_of_blocks(ds=ds, play=self, variable_manager=self._variable_manager, loader=self._loader)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/helpers.py", line 58, in load_list_of_blocks
    loader=loader
  File "/usr/lib/python2.7/site-packages/ansible/playbook/block.py", line 79, in load
    return b.load_data(data, variable_manager=variable_manager, loader=loader)
  File "/usr/lib/python2.7/site-packages/ansible/playbook/base.py", line 219, in load_data
    self._attributes[name] = method(name, ds[name])
  File "/usr/lib/python2.7/site-packages/ansible/playbook/block.py", line 115, in _load_block
    use_handlers=self._use_handlers,
  File "/usr/lib/python2.7/site-packages/ansible/playbook/helpers.py", line 181, in load_list_of_tasks
    raise AnsibleError("included task files must contain a list of tasks", obj=data)
NameError: global name 'AnsibleError' is not defined