Chap18 Creating the DQN: ValueError: Variables lengths do not match between Q network and target network

When I run the first chank of the code titled "Creating the DQN" in Chapter 18, I got the following error:

```
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
in ()
21 gamma=0.99, # discount factor
22 train_step_counter=train_step,
---> 23 epsilon_greedy=lambda: epsilon_fn(train_step))
24 agent.initialize()

6 frames
/usr/local/lib/python3.6/dist-packages/gin/config.py in gin_wrapper(*args, **kwargs)
1076 scope_info = " in scope '{}'".format(scope_str) if scope_str else ''
1077 err_str = err_str.format(name, fn_or_cls, scope_info)
-> 1078 utils.augment_exception_message_and_reraise(e, err_str)
1079
1080 return gin_wrapper

/usr/local/lib/python3.6/dist-packages/gin/utils.py in augment_exception_message_and_reraise(exception, message)
47 if six.PY3:
48 ExceptionProxy.__qualname__ = type(exception).__qualname__
---> 49 six.raise_from(proxy.with_traceback(exception.__traceback__), None)
50 else:
51 six.reraise(proxy, None, sys.exc_info()[2])

/usr/local/lib/python3.6/dist-packages/six.py in raise_from(value, from_value)

/usr/local/lib/python3.6/dist-packages/gin/config.py in gin_wrapper(*args, **kwargs)
1053
1054 try:
-> 1055 return fn(*new_args, **new_kwargs)
1056 except Exception as e: # pylint: disable=broad-except
1057 err_str = ''

/usr/local/lib/python3.6/dist-packages/tf_agents/agents/dqn/dqn_agent.py in __init__(self, time_step_spec, action_spec, q_network, optimizer, observation_and_action_constraint_splitter, epsilon_greedy, n_step_update, boltzmann_temperature, emit_log_probability, target_q_network, target_update_tau, target_update_period, td_errors_loss_fn, gamma, reward_scale_factor, gradient_clipping, debug_summaries, summarize_grads_and_vars, train_step_counter, name)
236 self._target_q_network = common.maybe_copy_target_network_with_checks(
237 self._q_network, target_q_network, input_spec=net_observation_spec,
--> 238 name='TargetQNetwork')
239
240 self._check_network_output(self._q_network, 'q_network')

/usr/local/lib/python3.6/dist-packages/tf_agents/utils/common.py in maybe_copy_target_network_with_checks(network, target_network, name, input_spec)
1281 # with the team.
1282 check_no_shared_variables(network, target_network)
-> 1283 check_matching_networks(network, target_network)
1284 return target_network
1285

/usr/local/lib/python3.6/dist-packages/tf_agents/utils/common.py in check_matching_networks(network_1, network_2)
1261 raise ValueError(
1262 'Variables lengths do not match between Q network and target network: '
-> 1263 '{} vs. {}'.format(network_1.variables, network_2.variables))
1264 for v1, v2 in zip(network_1.variables, network_2.variables):
1265 if v1.dtype != v2.dtype or v1.shape != v2.shape:

ValueError: Variables lengths do not match between Q network and target network: [, , , , , , , , , , ] vs. [, , , , , , , , , ]
In call to configurable 'DqnAgent' ()
```