Chiseled Bookshelfs emit a Signal of 1 despite having no Books

Chiseled Bookshelves emit a signal of 1-6, but when placing a book inside of a chiseled bookshelf and removing it, the bookshelf still emits a signal of one.

What I expected to happen was...:
When the bookshelf has no books in it, it should not emit a redstone signal.

What actually happened was...:
When placing and removing a book in a Chiseled Bookshelf, it still emits a redstone signal of one.

Steps to Reproduce:
1. Place a Chiseled Bookshelf and a Comparator connected to it
2. Place any number of books inside the Chiseled Bookshelf
3. Remove the Books in the Chiseled Bookshelf and check the signal.