Top half slab can't be placed on bottom half slab at sky limit level.

What I expected to happen was...:
Top half slab should be placed on top of bottom half slab, to form a full block.

In this case I have used Cobblestone slabs.

What actually happened was...
The game was checking for the building level and gave me an error every time, when actually I'm trying to fill up the block below.

Steps to Reproduce:
1. Build up to sky limit
2. finish with a bottom half slab

3. Try to complete the top with top half slab.