Extra Faces in obj file when exported from Sketchup

I’m having trouble identifying the problem with my model. When I export to an obj file, it has an extra face. Please tell me what I’m doing wrong! I’ve attached the skp file, and the obj and mtl files are in a zip folder.

ProVia 9-Lite w-Sidelites.skp (179.9 KB)
ProVia 9-Lite w-Sidelites.zip (81.1 KB)

I have identified the problem. The face needs to be split at the corners. I’m not sure why it causes a problem. Can anyone enlighten me?