Student Needs Help with Boolean Union

I’m trying to boolean union the arm of a design to the main body but instead it differences it. I checked that they were closed 3d objects. Both my instructor and an advanced student looked at it and no one can figure it out. Why is it differences instead of joining and how can I fix this?

Hi Emily- how are you determining that the objects are closed? At any rate, try Boolean2Objects and click to see the various possible outcomes until you like what you see, then Enter. If you are certain the objects are closed (ShowEdges reports no naked edges at the command line for example) then please send the two objects to tech@mcneel.com.

-Pascal