TypeError: object of type 'NoneType' has no len()

I followed [huggingface doc](https://huggingface.co/docs/transformers/v4.50.0/en/training#trainer) step by step, but get the following errors:
```
>>> trainer = Trainer(model=model, args=training_args, train_dataset=small_train_dataset, eval_dataset=small_eval_dataset, compute_metrics=compute_metrics)
>>> trainer.train()
33%|██████████████████████████████████████████████████▎ | 125/375 [12:53<25:46, 6.19s/it]
Traceback (most recent call last):
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/evaluate/module.py", line 518, in add_batch
if len(column) > 0:█████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 125/125 [00:35<00:00, 3.57it/s]
TypeError: object of type 'NoneType' has no len()

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "", line 1, in
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/transformers/trainer.py", line 2245, in train
return inner_training_loop(
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/transformers/trainer.py", line 2647, in _inner_training_loop
self._maybe_log_save_evaluate(tr_loss, grad_norm, model, trial, epoch, ignore_keys_for_eval, start_time)
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/transformers/trainer.py", line 3093, in _maybe_log_save_evaluate
metrics = self._evaluate(trial, ignore_keys_for_eval)
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/transformers/trainer.py", line 3047, in _evaluate
metrics = self.evaluate(ignore_keys=ignore_keys_for_eval)
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/transformers/trainer.py", line 4136, in evaluate
output = eval_loop(
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/transformers/trainer.py", line 4425, in evaluation_loop
metrics = self.compute_metrics(
File "", line 4, in compute_metrics
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/evaluate/module.py", line 455, in compute
self.add_batch(**inputs)
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/evaluate/module.py", line 523, in add_batch
if any(len(batch[c]) != len(next(iter(batch.values()))) for c in batch):
File "/Users/fanrong/transformers-env/lib/python3.9/site-packages/evaluate/module.py", line 523, in
if any(len(batch[c]) != len(next(iter(batch.values()))) for c in batch):
TypeError: object of type 'NoneType' has no len()
```
Could you please help me solve the problem? Thanks~