how to use ffmpeg on ubuntu to encode to dnxhr 12 bit

Question

i've been trying to make dnxhr 12 bit files using ffmpeg and it always seems to come out as 10 bit. when i've checked, it seems that my installed ffmpeg says it should be able to do this but ffprobe on the resultant file always says 10 bit. this is the version of ffmpeg i have:

ffmpeg version 3.4.6-0ubuntu0.18.04.1

ffmpeg -h encoder=dnxhd shows:

dnxhr_444 E..V....

if i process a dnxhr 444 12 bit file with this option:

-vcodec dnxhd -profile:v dnxhr_444 -pix_fmt rgb24

i get an error:

Incompatible pixel format 'rgb24' for codec 'dnxhd', auto-selecting format 'gbrp10le'

if i try to process the same file with this option:

-vcodec dnxhd -profile:v dnxhr_444 -pix_fmt yuv444p12le

i get this error:

Incompatible pixel format 'yuv444p12le' for codec 'dnxhd', auto-selecting format 'yuv444p10le'

i read someplace that others had these issues but i was under the impression that it was fixed. what am i doing wrong and what is the correct option to create dnxhr 444 12 bit using ffmpeg?

thanks, babag