I am performing MD simulation for drug-DNA complex using GROMACS via AMBER force field.

At following step, the program shows an error:

gmx genion -s ions.tpr -o solvated.pdb -p topol.top -pname NA -np 22

The error is:

Fatal error:

Your solvent group size (19007) is not a multiple of 28

Please let me know what are possible mistakes that I'm making?