TypeError while running data_reader.py

[CarolLi]

Environment
allennlp == 0.9.0
python == 3.6.12
torch == 1.5.0

I ran data_reader.py and it occurred an error as following,

Using backend: pytorch
Traceback (most recent call last):
File "data_reader.py", line 2, in
dataset_reader = data_reader.CNNDMDatasetReader()
File "/home/carol/DiscoBERT/model/data_reader.py", line 114, in init
self._token_indexers = token_indexers['bert'] or {"tokens": SingleIdTokenIndexer()}
TypeError: 'PretrainedBertIndexer' object is not subscriptable

And I found that there wasn't a key named 'bert' in token_indexers, that's why the problem occurred. I wonder If the bert model has been correctly obtained by the following code.
token_indexers: Dict[str, TokenIndexer] = PretrainedBertIndexer("bert-base-uncased")

And here is the content in token_indexers,

token_indexers: {'_token_min_padding_length': 0, 'vocab': OrderedDict([('[PAD]', 0), ... , ('##～', 30521)]), 'wordpiece_tokenizer': <bound method WordpieceTokenizer.tokenize of <pytorch_pretrained_bert.tokenization.WordpieceTokenizer object at 0x7f008d3d84a8>>, '_namespace': 'bert', '_added_to_vocabulary': False, 'max_pieces': 512, 'use_starting_offsets': False, '_do_lowercase': True, '_truncate_long_sequences': True, '_warned_about_truncation': False, '_never_lowercase': {'[SEP]', '[PAD]', '[UNK]', '[CLS]', '[MASK]'}, '_start_piece_ids': [101], '_end_piece_ids': [102], '_separator_ids': [102]}