Monsters pathfinding stops working when their head intesects a ladder

Steps to reproduce: 

1: place a 2 block high pillar of stone

2:place a ladder on the the side of the top block of stone

3: spawn zombie below ladder

4: observe

Expected result: after a short period of time zombie ambulates to a nearby block

Observed result: zombie sits still intersecting with the ladder until the heat death of the universe 

Bug