Comparators not turning off

Put the summary of the bug you're having here

What I expected to happen was...:
i expected to have the comparator turn off when there were not any books in the chizeled bookshelf

What actually happened was...:
the comparator did not power off when there weren't any books in the chizeled bookshelf

Steps to Reproduce:
1. Put down a chizeled bookshelf
2. Place a comparator on the ground facing away so that it receives signal from the chizeled bookshelf
3. Put one book in and then take out the book. Then you have a signal which wont go away.