KeyError: 'priority' when "$ populus deploy --chain horton Greeter"

* Populus Version: 1.6.9
* OS: osx

### What was wrong?
I'm doing Part 2:Local Chain of this site http://populus.readthedocs.io/en/latest/tutorial.part-2.html
But error has occured. I'm not sure how fix it.

* full output
> Found 1 contract source files
- contracts/Greeter.sol
> Compiled 1 contracts
- Greeter
Traceback (most recent call last):
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/config/base.py", line 135, in __getitem__
return self.resolve(get_nested_key(self._wrapped, key))
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/utils/mappings.py", line 46, in get_nested_key
return getter_fn(config)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/eth_utils/functional.py", line 11, in
return lambda x: f(g(x))
KeyError: 'priority'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Users/xxx/.pyenv/versions/3.5.1/bin/populus", line 11, in
sys.exit(main())
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/click/core.py", line 722, in __call__
return self.main(*args, **kwargs)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/click/core.py", line 1066, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/click/decorators.py", line 17, in new_func
return f(get_current_context(), *args, **kwargs)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/cli/deploy_cmd.py", line 90, in deploy_cmd
provider = chain.provider
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/chain/base.py", line 140, in provider
if not self.provider_backends:
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/eth_utils/functional.py", line 22, in inner
return callback(fn(*args, **kwargs))
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/chain/base.py", line 134, in provider_backends
for backend_name, backend in self.contract_backends.items():
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/utils/functional.py", line 27, in __get__
res = instance.__dict__[self.name] = self.func(instance)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/eth_utils/functional.py", line 22, in inner
return callback(fn(*args, **kwargs))
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/chain/base.py", line 121, in contract_backends
for backend_name, backend_config in self.contract_backend_configs.items():
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/eth_utils/functional.py", line 22, in inner
return callback(fn(*args, **kwargs))
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/chain/base.py", line 113, in contract_backend_configs
self.project.config
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/eth_utils/functional.py", line 22, in inner
return callback(fn(*args, **kwargs))
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/utils/config.py", line 107, in sort_prioritized_configs
in resolved_backend_configs
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/collections/__init__.py", line 514, in __init__
self.update(*args, **kwds)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/collections/__init__.py", line 601, in update
_count_elements(self, iterable)
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/utils/config.py", line 106, in
for backend_name, config
File "/Users/xxx/.pyenv/versions/3.5.1/lib/python3.5/site-packages/populus/config/base.py", line 137, in __getitem__
raise KeyError("Key '{0}' not found in {1}".format(key, self._wrapped))
KeyError: "Key 'priority' not found in {}"

* what command you ran
$ populus deploy --chain horton Greeter

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