# Buggy code
def __init__(self, component_config: Optional[Dict[Text, Any]] = None) -> None:
    super(Featurizer, self).__init__(component_config)

    try:
        self.return_sequence = self.component_config["return_sequence"]
    except KeyError:
        raise KeyError(
            "No default value for 'return_sequence' was set. Please, "
            "add it to the default dict of the featurizer."
        )


# Traceback (KeyError)
Traceback (most recent call last):
File "/home/xxxx/anaconda3/envs/ChatAna/bin/rasa", line 8, in <module>
sys.exit(main())
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/__main__.py", line 76, in main
cmdline_arguments.func(cmdline_arguments)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/cli/train.py", line 76, in train
kwargs=extract_additional_arguments(args),
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/train.py", line 46, in train
kwargs=kwargs,
File "uvloop/loop.pyx", line 1456, in uvloop.loop.Loop.run_until_complete
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/train.py", line 97, in train_async
kwargs,
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/train.py", line 184, in _train_async_internal
kwargs=kwargs,
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/train.py", line 241, in _do_training
persist_nlu_training_data=persist_nlu_training_data,
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/train.py", line 470, in _train_nlu_with_validated_data
persist_nlu_training_data=persist_nlu_training_data,
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/train.py", line 74, in train
trainer = Trainer(nlu_config, component_builder)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/model.py", line 147, in __init__
self.pipeline = self._build_pipeline(cfg, component_builder)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/model.py", line 159, in _build_pipeline
component = component_builder.create_component(component_cfg, cfg)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/components.py", line 482, in create_component
component = registry.create_component_by_config(component_config, cfg)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/registry.py", line 232, in create_component_by_config
return component_class.create(component_config, config)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/components.py", line 267, in create
return cls(component_config)
File "/home/xxxx/anaconda3/envs/ChatAna/lib/python3.6/site-packages/rasa/nlu/tokenizers/tokenizer.py", line 60, in __init__
"No default value for 'use_cls_token' was set. Please, "
KeyError: "No default value for 'use_cls_token' was set. Please, add it to the default dict of the tokenizer."

# Fixed code
def __init__(self, component_config: Optional[Dict[Text, Any]] = None) -> None:
    super(Featurizer, self).__init__(component_config)

    try:
        self.return_sequence = self.component_config["return_sequence"]
    except KeyError:
        warnings.warn(
            "No default value for 'return_sequence' was set. Please, "
            "add it to the default dict of the featurizer and set it to 'False'."
        )
        self.return_sequence = False