[gmx-users] Problem with gmx distance

[Asmaa El khodary]

Hi,
I created an index files with two residue groups, 15 "r 22" contains 15
atoms and 16 "r 137" contains 24 atoms. Then, I use:
gmx distance -s run.tpr -f run.xtc -n index.ndx -oall dist.xvg
when I try to select both groups, I get this message
Cannot match 'group "15"', because no such index group can be found.
Could you explain that error to me please? How to fix it?
Thanks
Asmaa