Stairs let in light

Stairs let in light;they should not.

What I expected to happen was...:
I expect no light to be let through, since stairs are opaque.

What actually happened was...:
A bit of light seeps in. Note if smooth lighting is turned off this bug does not occur. It only occurs with smooth lighting on.

Steps to Reproduce:
In a light area, enclose yourself in a house made of wooden stairs. It should be completely dark. Observe the lighting glitch.