ValueError: 63 is not in list

[deepbluesea]

tensor flow 2.0.0-beta1

InvalidArgumentError: ValueError: 63 is not in list
Traceback (most recent call last):

  File "/usr/local/lib/python3.6/dist-packages/tensorflow/python/ops/script_ops.py", line 209, in __call__
    ret = func(*args)

  File "/usr/local/lib/python3.6/dist-packages/tensorflow/python/data/ops/dataset_ops.py", line 525, in generator_py_func
    values = next(generator_state.get_iterator(iterator_id))

  File "/content/gpt2-estimator/gpt2_estimator/gpt2_estimator_fn.py", line 118, in sample_batch
    mask_start = len(v) - list(v[::-1]).index(63) + 1

ValueError: 63 is not in list