If the aperture is constant for all raws of the merge, its value shouldn’t play a role (with the exception of aperture 0 in your case, which seems to lead to division by zero). Unfortunately Canon cams don’t allow to set the aperture (even max aperture would be useful) for manual lenses. I understand that it’s not possible for Canon cameras to store the current aperture in exif when using manual lenses, but storing the max aperture maybe would be possible technically and probably help in this case.