[Rivet] Rivet with POWHEG-BOX

[Tomas Jezo]

Dear Rivet authors,

I'm trying to analyze events generated by POWHEG-BOX using Rivet (rivet 
v2.4.3). I'm using the `HepMC::IO_HEPEVT` to convert the events from the 
HEPEVT format and them pass them to rivet:

HepMC::GenEvent* evt = hepevtio.read_next_event();
rivet.analyze(*evt);

Unfortunately i get the following error:

terminate called after throwing an instance of 'Rivet::PidError'
   what():  Particle ID '21' not known.

starting from event 2 (event 1 succeeds).

If I write events into a file I get:

HepMC::Version 2.06.09
HepMC::IO_GenEvent-START_EVENT_LISTING
E 1 -1 -1.0000000000000000e+00 -1.0000000000000000e+00 -1.0000000000000000e+00 0 0 1 1 2 0 1 1.1406100000000000e+01
N 1 "0"
U GEV MM
V -1 0 0 0 0 0 2 4 0
P 1 21 0 0 3.4591526030000000e+02 3.4591526030000000e+02 0 -1 0 0 -1 0
P 2 21 0 0 -4.2588502149999999e+02 4.2588502149999999e+02 0 -1 0 0 -1 0
P 3 6 -1.4833769729999999e+00 -1.2369079390000000e+02 2.3239638199999999e+02 3.1474742459999999e+02 1.7250000000000000e+02 1 0 0 0 0
P 4 -6 -1.2441412769999999e+01 9.4068674470000005e+01 -3.5206476490000000e+02 4.0337272209999998e+02 1.7250000000000000e+02 1 0 0 0 0
P 5 5 7.2436624820000004e+00 2.2627455149999999e+01 3.3183952779999998e+01 4.1087827820000001e+01 4.7500000000000000e+00 1 0 0 0 0
P 6 -5 6.6811272600000002e+00 6.9946643320000002e+00 6.5146689540000002e+00 1.2592307260000000e+01 4.7500000000000000e+00 1 0 0 0 0
E 2 -1 -1.0000000000000000e+00 -1.0000000000000000e+00 -1.0000000000000000e+00 0 0 1 1 2 0 1 1.1406100000000000e+01
N 1 "0"
U GEV MM
V -1 0 0 0 0 0 2 4 0
P 1 21 0 0 1.5871885349999999e+01 1.5871885349999999e+01 6.7434957620000004e-07 -1 0 0 -1 0
P 2 21 0 0 -2.1900844820000002e+03 2.1900844820000002e+03 0 -1 0 0 -1 0
P 3 6 -1.3328056880000000e+01 -8.3731995950000009e+00 -7.8224201930000004e+02 8.0119072280000000e+02 1.7250000000000000e+02 1 0 0 0 0
P 4 -6 2.1782820879999999e+01 1.1731505100000000e+01 -1.3307128740000001e+03 1.3420749320000000e+03 1.7250000000000000e+02 1 0 0 0 0
P 5 5 -3.3212752330000002e+00 -2.3273609810000000e+00 -2.7406078330000000e+01 2.8108772790000000e+01 4.7500000000000000e+00 1 0 0 0 0
P 6 -5 -5.1334887599999997e+00 -1.0309445209999999e+00 -3.3851624520000001e+01 3.4581939450000000e+01 4.7500000000000000e+00 1 0 0 0 0
....

Same happens when I run rivet from the command line:
1.) It succeeds when analyzing individual events (event 1 or event 2)
2.) Fails if run with a file containing both

What am I doing wrong? Would you have any advice for me?

Many thanks.

Cheers,
Tomas