I've been working on a ship hull and have been doing it in halves, then mirroring the body. Every time I try and combine the features, I keep getting zero-thickness errors and it shows some lines highlighted. I can't seem to find ANYTHING wrong with the bodies though.

I did this with the bottom half and the combine feature worked perfectly, using the same pierce points for the centerline. The upper half doesn't seem to want to work.

Help me keep my sanity before I end up putting my fist through my screen.