allennlp.common.checks.ConfigurationError: calculate_span_f1 is True, but no label_encoding was specified.

[SongChujun]

running python example.py but got the following error

Traceback (most recent call last):
  File "test.py", line 5, in <module>
    archive = load_archive("wiki-truecaser-model-en.tar.gz")
  File "/Users/song/anaconda3/lib/python3.7/site-packages/allennlp/models/archival.py", line 197, in load_archive
    opt_level=opt_level,
  File "/Users/song/anaconda3/lib/python3.7/site-packages/allennlp/models/model.py", line 398, in load
    return model_class._load(config, serialization_dir, weights_file, cuda_device, opt_level)
  File "/Users/song/anaconda3/lib/python3.7/site-packages/allennlp/models/model.py", line 295, in _load
    model = Model.from_params(vocab=vocab, params=model_params)
  File "/Users/song/anaconda3/lib/python3.7/site-packages/allennlp/common/from_params.py", line 580, in from_params
    **extras,
  File "/Users/song/anaconda3/lib/python3.7/site-packages/allennlp/common/from_params.py", line 611, in from_params
    return constructor_to_call(**kwargs)  # type: ignore
  File "/Users/song/Desktop/nmt/joeynmt/chatnmt/code/pytorch-truecaser/mylib/truecaser.py", line 26, in __init__
    super(TrueCaser, self).__init__(vocab, text_field_embedder, encoder, initializer, regularizer)
  File "/Users/song/anaconda3/lib/python3.7/site-packages/allennlp/models/simple_tagger.py", line 100, in __init__
    "calculate_span_f1 is True, but no label_encoding was specified."
allennlp.common.checks.ConfigurationError: calculate_span_f1 is True, but no label_encoding was specified.