Rotacf indexing problem

I am unable to combine two labels in my index for the -d vector label for rotacf.

I have tried ( 12 , 13) , 12 13 ,( 12, 13) for example in my index file but all are having same errors.
Program: gmx rotacf, version 2022.1
Source file: src/gromacs/gmxana/gmx_rotacf.cpp (line 138)

Fatal error:
number of index elements not multiple of 2, these can not be atom doublets

For more information and tips for troubleshooting, please check the GROMACS
website at Common Errors — GROMACS webpage documentation


Could you please share the input files and the exact command you used?