Error with xz

[Robert Moskowitz]

First I dd an SD card:

sudo dd if=/dev/sdb of=/home/rgm/arm/rsel-minimal.img bs=4096 
conv=notrunc,noerror

Then I ran xz which produced an error:

xz -k rsel-minimal.img
xz: rsel-minimal.img.xz: Cannot set the file group: Operation not permitted

Is this because the sudo made the owner of .img root?  Do I ignore the 
error or go back and change the owner of .img?

I will want to make this .img available to others to download and build 
their own SDcards for booting on Cubieboards.