Group Water_and_ions referenced in the .mdp file was not found in the index file

GROMACS version:
GROMACS modification: Yes/No
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Here I am getting only 17 items in the list but this runs fine after that I used the command "gmx grompp -f nvt.mdp -c em.gro -r em.gro -p -n index.ndx -o nvt.tpr
" which is throwing the error