Subtract Error

Hello there!

I can’t understand why the subtract tool tells me that the subtraction failed, because the new body wouldn’t be manifold. The height of the smaller piece is 2mm less than the bigger one. The underside of the smaller piece is on the same height than the underside of the bigger one, so when printed i can open it to the underside.

The strange thing is, when I lift the smaller piece one mm, so it’s completely enclosed by the bigger part, the subtraction works perfectly fine.

I’m not sure what I’m doing wrong?!