Can place trapdoors on the top half or bottom half of solid blocks but when it comes to half-slabs it is only possible to place on an upper slab.

What I expected to happen was...:
The creation of a (vertically) shuttered window by placing on the top slab of one block and the bottom slab of the one below it.

What actually happened was...:
The creation of some weird ugly window shade. ie: simply did not place the bottom half

Steps to Reproduce:
1. Place a lower half slab
2. Attempt to attach a trapdoor to it
3. Fail