Fisheye Chess calibration with fix pixel aspect ration of 1 - HOW?

I am using a wide-angle lens and the Fisheye model works the best for me.
when I doing the chess calibration with the Fisheye class I can’t find a flag that forces the pixel to have an aspect ratio of 1.
for the standard model calibration, I do have a flag of “CALIB_FIX_ASPECT_RATIO” but not for the fisheye.
any idea?