Hi @krwin, this indeed seems to be a bug in the notebook 5, where the order of the arguments for tokenizer.train() in this cell
from tokenizers.trainers import BpeTrainer

# We initialize our trainer, giving him the details about the vocabulary we want to generate
trainer = BpeTrainer(vocab_size=25000, show_progress=True, initial_alphabet=ByteLevel.alphabet())
tokenizer.train(trainer, ["big.txt"])

print("Trained vocab size: {}".format(tokenizer.get_vocab_size()))

is back-to-front (see the docs 6). To fix the problem you can just specify the arguments explicitly:
tokenizer.train(trainer=trainer, files=["big.txt"])

cc: @anthony