C++ SDK - image mask for transparency; special requirements?

I have a PDEImage that I assign to a PDEImage with the simple call:

PDEImageSetSMask(pdeImage, pdeImageMask);

and then I get an error in Acrobat that says something is wrong and the page may not display correctly and the image does not show. Don't do the assignment and it is fine. But I can change my code so that I use the image mask I ceate as the image and that displays fine; it is a grayscale rendering of the same image. So it is a valid PDImage but I assume it must not meet some special requirement to be a mask. I am having trouble finding any concise information on image masks. Is there somewhere it is documented. It's another case where I am using a working sample as the basis for my code. As mentioned, my own experiemnt of just using the generated mask as teh image demonstrates that it is a properly formed PDImage. but something makes it fail when used a s a mask.