Clipping Mask Not Preserved when exporting from .AI to .PSD

Hi,

When I create a clipping mask in an .AI file, and then export that document to a PSD, the PSD contains the clipped out areas, including them as transparency.   I have adapted to this by trimming out the transparency in .PSD.  But for a particular project batch, I cannot trim them for other reasons. 

I'm running  Adobe Illustrator CS5.1 on OS X 10.7.5

Any help is much appreciated.

Tom