ISIS - problem with subtracting images

[Robin Leadbeater]

Does anyone else have problem using the ISIS image subtraction function "add/subtract an image by another" in "image processing 1" ?
I get the wrong answer if I subtract a master offset image generated by ISIS from a raw spectrum image direct from the camera. (It seems to subtract 1/2 the correct offset.)
I think it might be caused by mixing 16 and 32 bit images but I get the correct result if I use the "Preprocessing" function in "image processing 2" and of course it works ok if I process the image using the "General" tab

Thanks
Robin