LHE reader stalls when used by Rivet

I don't know if this is a Rivet or HepMC issue, but when I try to read an LHE file directly into Rivet as follows, then HepMC3 first recognises it as LHE but stalls after the following lines:

$ rivet -a MC_ZINC unweighted_events.lhe
Rivet 3.1.7 running on machine tauruslogin3.taurus.hrsk.tu-dresden.de (x86_64) at 2023-01-31 11:21:08
DEBUG(0)::deduce_reader: Attempt ReaderRootTree for unweighted_events.lhe
DEBUG(0)::Attempt ReaderAscii for unweighted_events.lhe
DEBUG(0)::Attempt ReaderAsciiHepMC2 for unweighted_events.lhe
DEBUG(0)::Attempt ReaderLHEF for unweighted_events.lhe

The versions are Rivet 3.1.7 and HepMC 3.02.04.

The file to reproduce this can be found in: https://cern.ch/fsiegert/tmp/unweighted_events.lhe