I am trying to build a topology in GROMACS for NMA (N-methyl acetamide). I need to use one of the GROMOS force fields. I added my own rtp file to the force field for describing the NME residue, which is not in the force field originally. When I run pdb2gmx, I get the error: "Residues in one molecule have a different 'bonds' type: 2 and 1". What is causing this inconsistency? What can I do to move forward?