Gmx angle error

GROMACS version: 2016.4
GROMACS modification: No

Hi all,

I’m performing simulations with a coarse-grained bead and spring model of a polymer and want to measure the average of each dihedral in the system. I use:

gmx_mpi angle -f traj_comp.xtc -n index.ndx -ov -type dihedral

for several systems. One of them brings up the following error:

Fatal error:
angle (-nan) index out of range (0…360) : -214748364

but I’m unsure about what’s happened because I checked my index file and there are no issues and I’ve used the same method for every other system and there’s been no problem. Also every time I try the analysis, the error comes up at different times along the trajectory. Sometimes it happens straight away and others after going through 2 microseconds of the trajectory. Has anyone come across this before and know what it means?