Can't place Jungle Wood Slab onto Oak Wood Slab

I tried to build a house with Oak and Jungle Wood. The ground floor was built with Oak Wood and now I tried to build the 1. floor. There the ground should be build with jungle wood, but the top of the ground floor should be Oak wood. So I tried to place at first an Oak wood slab and onto this a jungle wood slab, but I can't place the 2nd slab directly onto the other.