Found a strange thing that the BZERO & BSCALE cards can't be written to the header by the code below

hdu = fits.open(fits_file)

h = hdu[0].header

img = hdu[0].data

h['BZERO'] = 0.

h['BSCALE'] = 1.

fits.writeto(fits_file,img,h)

This will cause problems when using getsources, since it reads these cards. A solution is (though I don't know why)

hdu = fits.open(fits_file)

hdu[0].header['BZERO'] = 0.

hdu[0].header['BSCALE'] = 1.

fits.writeto(fits_file,hdu[0].data,hdu[0].header)