`grdimage`, request to scale the pallete via `-C...+Uk` seems ignored

[mkononets]

grdimage, request to scale the pallete via -Cgeo+Uk seems ignored.
The expected result would be the colorbar with tick labels in kilometers. Now it is in meters regardless of the +Uk

gmt grdimage @earth_relief_10m -Cgeo+Uk -R110/155/-45/-10 -JM10c -B -png Australia

Australia.png: