Hans-Gerd,
Thank you for the solution. It took me a bit to figure out what you did, and I was able to replicate it. I also had to tinker a bit with the settings, since my shape is a 180 degree sweep instead of 90. One question - why did you need the inner group? I found that by counter rotating the shape itself, I could achieve the same effect with fewer objects. I've attached my modified version of your example with documentation in case others run into the same situation.
masking_.hype.zip (60.5 KB)
Thanks for pointing me in the right direction.
Art.