Furnace not working

I cannot smelt anything in my furnace.

What I expected to happen was...:
I put an item in the top box in the furnace, iron ore for example then I am supposed to be able to fuel my furnace by placing fuel in the bottom box thus creating a new item.

What actually happened was...:
After placing an item in the top box in the furnace, I am not able to place fuel in the bottom box. I cannot create any new items. Therefore I cannot get very far in my game.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...