Thanks for the tip @lhoestq, but converting to lists leads to another error message
---------------------------------------------------------------------------
ArrowTypeError                            Traceback (most recent call last)
<ipython-input-55-67d4df0cb97c> in <module>
----> 1 bleu_metric.compute(preds[0].tolist(), targets.tolist())

/opt/conda/envs/fastai/lib/python3.7/site-packages/nlp/metric.py in compute(self, predictions, references, timeout, **metrics_kwargs)
    191         """
    192         if predictions is not None:
--> 193             self.add_batch(predictions=predictions, references=references)
    194         self.finalize(timeout=timeout)
    195 

/opt/conda/envs/fastai/lib/python3.7/site-packages/nlp/metric.py in add_batch(self, predictions, references, **kwargs)
    207         if self.writer is None:
    208             self._init_writer()
--> 209         self.writer.write_batch(batch)
    210 
    211     def add(self, prediction=None, reference=None, **kwargs):

/opt/conda/envs/fastai/lib/python3.7/site-packages/nlp/arrow_writer.py in write_batch(self, batch_examples, writer_batch_size)
    155         if self.pa_writer is None:
    156             self._build_writer(pa_table=pa.Table.from_pydict(batch_examples))
--> 157         pa_table: pa.Table = pa.Table.from_pydict(batch_examples, schema=self._schema)
    158         if writer_batch_size is None:
    159             writer_batch_size = self.writer_batch_size

/opt/conda/envs/fastai/lib/python3.7/site-packages/pyarrow/types.pxi in __iter__()

/opt/conda/envs/fastai/lib/python3.7/site-packages/pyarrow/array.pxi in pyarrow.lib.asarray()

/opt/conda/envs/fastai/lib/python3.7/site-packages/pyarrow/array.pxi in pyarrow.lib.array()

/opt/conda/envs/fastai/lib/python3.7/site-packages/pyarrow/array.pxi in pyarrow.lib._sequence_to_array()

/opt/conda/envs/fastai/lib/python3.7/site-packages/pyarrow/error.pxi in pyarrow.lib.check_status()

ArrowTypeError: Expected a string or bytes object, got a 'list' object