Unable to add floor/ceiling in room builder

[Gbrec]

For some reason I'm unable to add a floor and/or ceiling in the current set. Even if I draw a square of walls in the room builder and add a ceiling I get the message: "No Room Found" and telling me that it has 4 walls but it's not enclosed in a room.

The problem is only in the current set I'm working. I exported the set and imported it in a new file, same problem. There seems to be something there that blocks this function. The set only contains walls and doors.

Anyone else seeing this problem?

FrameForge Version: 3.6.0.14
MacOS: 10.11.1