Trouble with Normals and alpha channels

I have a microwave door which I am trying to add a crack onto the window door. There are three image layers in the material: The stripes of glass door, the crack image, and a normal map for the crack. Both the crack image and the normal map have alpha channels. However it appears that the alpha channel for the normal map is not acting correctly (hence the black box on the scene microwave window). enclosed is a picture that shows what is being rendered. Any help would be much appreciated. Thanks!

Normal maps don’t use alpha channels. If your normal map is set to affect normal influence and nothing else, your problem lies elsewhere.