Bolean problem

I want to make a rectangular “cave” in my wall (not completely trough the wall) but there seem to be a problem i can not fix.I select my wall,then i select the bolean modifier,i select the cube and then difference.But once i hit apply the “cave” is not concave but convex.I made my window like this and had no problems.

Look at my screenshot,i hope you will understand what i mean.

For the cube, did you press Ctrl-A to reset the rotation and scale before you applied the boolean? Also, did you make sure that the cube’s normals were on the outside?