I’m trying to generate a pyparsing grammar railroad diagram, as described here, but when I run the example script the render comes out wrong with all these black boxes.

Script:

from pyparsing import Word, nums, alphas

street_address = Word(nums).set_name("house_number") + Word(alphas)[1, ...].set_name("street_name")
street_address.set_name("street_address")
street_address.create_diagram("street_address_diagram.html")

Render:

(To install, I ran pip install "pyparsing[diagrams]")

What am I doing wrong?