Items dissapear in furnace

If you place a redstone comparator facing awa from a furnace and put any item to smelt in the furnace it will dissapear.

What I expected to happen was...:
the items do not dissapear

What actually happened was...:
the items dissapear

Steps to Reproduce:
1. you place a furnace down
2. you place a redstone comparator next to the furnace
3. you put any item to smelt