I'm shelling a semi-complex feature with a thru hole for a screw. The outer thru hole is 3mm from the outer wall of the body. When I shell the part at 2mm, it completes the operation; however, it puts the wall for the thru hole into the wall of the overall body without combining the two into one 3mm thick wall. 

I tried extruding more material to bridge the two walls, but it said it was "creating zero thickness geometry" or "failed due to geometric condition." 

Does anyone know how to prevent this?