Terraria Merchant wont spawn?

I have 1 room that isn't being used by any other npc's, it has no open areas and the walls are made of wood, as are the background walls. I have 4 chairs and 2 tables, alot of torches (more than 10), and 2 doors. I have more than 50 silver in my inventory and the merchant wont spawn. What am I doing wrong?