Mobs unable to move in slabbed house

What I expected to happen was...:
In my house made 2 blocks high but with slabbed roof and floor to save blocks villagers would be able to move around like normal

What actually happened was...:
Villagers were unable to move as were zombies

Steps to Reproduce:
1. Create frame 3 high with a gap in middle (as seen in attached recreation pictures).
2. Place slab at bottom and upside down slab on top.
3. Walk trough