class_names need to be a list not a numpy.ndarray

[saljamil]

Page 27 of the book - had to change the code to this:
fig, ax = plt.subplots(figsize=(8, 4))
tree.plot_tree(stump, feature_names=['value'],
filled=True,
class_names=list(stump.classes_),
ax=ax)

with class_names param needing a list