Why doesn't work convert to fits from cr2?

[Mondi]

nconvert -o file.fits -out fits file.cr2

I get the error message

Error: This picture cannot be written using this format (file.fits)

[xnview]

nconvert -o file.fits -out fits file.cr2

I get the error message

Error: This picture cannot be written using this format (file.fits)

FITS support only greyscale, so add -grey 256