Here, I created a 6-polygon .obj cube, 2x2x2 Poser units sides, centred at (0,0,0), and imported it into Poser. Seen with the dolly camera at (0,0,0), yaw = 45deg, pitch = 35deg up, so it sees an upper corner of the cube from the inside. The cube is "inside out", i.e. the "right sides" of the faces are inside = towards the camera. Rendered with Firefly in "cast shadows" mode. The render makes this image whether or not the edges of the cube are welded.

Superfly produces a solid black image.