The book does not indicate to add the price to the fixture, so this is entirely my mistake. But the reader, which has to keep up with a lot of details might think I should have added this earlier and add it. Running the tests laters results in errors.
I changed the paragraph following this code to mention this:
Note that if you didn’t choose to do the optional exercises in Playtime, on page 141, you need to modify all of the references to products and carts at this time and not add price to the line items.