Mask layer with alpha-blending

I have to layers where the top acts as a mask for the bottom.

It works fine, except it doesn’t work when I have something with alpha-blending in my mask layer. It does show it, but not with alpha 50 % which something in the mask layer is.

I remember sometime that it was something about the blending options with the layers (layer, alpha etc.) but I can get it to work. Any help?