Wooden slabs will not break when using an axe

Every time I need to take up a wooden slab, I need a Pickaxe to collect it without destroying it.

What I expected to happen was...:
I thought that I would use an axe to collect a wooden slab.

What actually happened was...:
I needed a Pickaxe to collect it.

Steps to Reproduce:
1. Place a wooden slab
2. Take out an axe and try to collect it.
3. Take out a Pickaxe, compare, and collect it.