Route book.render (path /book) does not seem to work

To reproduce:

1. Enable the Book module and the Help module.
2. Logged in as user 1 (or a user with book and node admin permission) Go to Create Content. Create a book node, then click "Add child page", and add a child. The book seems to be OK so far - you can see the book and its parent.
3. From either page, click "Outline". Help at the top of the page says "The outline feature allows you to include pages in the Book hierarchy, as well as move them within the hierarchy or to reorder an entire book.". "Book hierarchy" is a link to the route book.render, which is at path /book. "reorder an entire book" is a link to the route book.admin, which is at path /admin/structure/book.
4. Click the /book link. You get an error page:

Notice: Undefined index: href in Drupal\book\Controller\BookController->bookRender() (line 108 of core/modules/book/lib/Drupal/book/Controller/BookController.php).
Notice: Undefined index: options in Drupal\book\Controller\BookController->bookRender() (line 108 of core/modules/book/lib/Drupal/book/Controller/BookController.php).
Recoverable fatal error: Argument 3 passed to l() must be of the type array, null given, called in (.....)core/modules/book/lib/Drupal/book/Controller/BookController.php on line 108 and defined in l() (line 1025 of core/includes/common.inc).