Problem with boolean subtract

 From:  mickelsen
3330.1 
I have created solid object using the sweep command. It is the winding, rectangular-cross-section thingy in the drawing. I then created a cylinder using extrude. I want to subtract the cylinder from the thingy but when I do, it takes out the cylinder and the entire left side of the thingy. I don't know why. How can I get it to just take out the cylindrical portion of the thingy but leave the rest of both sides?
Thanks,
Mark