I just ran the notebook, latest version of fastai2 libs (fastai2, fastcore, course-v4 repos), and cell #12 [subword(1000)] ran fine. Can you check your versions of fastcore/fastai2/sentencepiece, and update/upgrade where needed, then try again?
Some of my relevant library versions listed below. Note that my fastcore and fastai2 are both with ‘editable pip install’, and are the latest version (having just done git pull) as of right now. sentencepiece seems to have a 0.1.90 (on their Github), but mine is older.
Yijin
Name: fastcore
Version: 0.1.18
Name: fastai2
Version: 0.0.18
Name: sentencepiece
Version: 0.1.83