Joining exterior wall with interior wall

[easplund]

I am trying to model a typical southwestern style house consisting of a central section with a peaked roof and cathedral ceilings flanked by two flat-roofed wings. I created a second floor with three rooms, one above each section. The rooms for the flat sections have no roof or ceiling, and that above the central peaked section has no ceiling and is open to below. Since I want the walls dividing these three sections to be full gable walls, I made them exterior walls. They are place directly above the interior walls of the first floor central room. However, when I take a 3D camera view of the interior of the greatroom, there is a horizonal line between the first floor and second floor walls. How do I get rid of this line?