The furnace is not working

Put the summary of the bug you're having here

What I expected to happen was...:
I thougt I could make iron ingots and charcoal.

*What actually happened
The furnace would not let me put in fuel

Steps to Reproduce:
1.make a furnace
2. Put anything you can use in the furnace in the top slot
3. It will not let you put fel in the bottom slot