Hasta La Motion!

I’m new here, and I’m just starting to learn Nuke (version 6). I have a question about the rotopaint node. I made two shapes in the rotopaint node, and now I want to substract one from the other, so I set the blending mode of that shape to ‘minus’. When I use the node on the side, as a mask input of the merge node, this works. However, when I use the node inline with the image, I get a strange behaviour.

To make things clear I have made screenshots of the situations. The first one shows what happens when I place the node inline with the image. The second one shows what happens when I use it as a mask input on the side.

Am I doing something wrong?

Hans Willem