diff --git a/src/Core/Event.cc b/src/Core/Event.cc
--- a/src/Core/Event.cc
+++ b/src/Core/Event.cc
@@ -1,58 +1,57 @@
 // -*- C++ -*-
 #include "Rivet/Event.hh"
 #include "Rivet/Tools/BeamConstraint.hh"
 #include "Rivet/Tools/Logging.hh"
 #include "Rivet/Tools/Utils.hh"
 #include "Rivet/Projections/Beam.hh"
 #include "HepMC/GenEvent.h"
 
 namespace Rivet {
 
   double Event::weight() const {
     // Get the weight index, which defaults to 0, i.e. nominal
-    // NB. this should normally only perform the slow env var lookup once per run
-    static ssize_t WEIGHT_INDEX = -1;
-    if (WEIGHT_INDEX < 0) {
+    // NB. This should normally only perform the slow env var lookup once per run
+    // NB. An explicitly set index of -1 is used to ignore all weights, for debugging
+    static int WEIGHT_INDEX = -2;
+    if (WEIGHT_INDEX < -1) {
       WEIGHT_INDEX = getEnvParam<size_t>("RIVET_WEIGHT_INDEX", 0);
       Log::getLog("Core.Weight") << Log::TRACE << "Got weight index from env/default = "<< WEIGHT_INDEX << endl;
     }
-    // If the env var weight index really is -1, or there are no event weights, return 1
-    // NB. setting RIVET_WEIGHT_INDEX=-1 forces an environment lookup every time: SLOW!
-    // But it's an edge case, so ok (?)
+    // If RIVET_WEIGHT_INDEX=-1, or there are no event weights, return 1
     if (WEIGHT_INDEX == -1 || genEvent()->weights().empty()) return 1.0;
     // Otherwise return the appropriate weight index
     return _genevent.weights()[WEIGHT_INDEX];
   }
 
   double Event::centrality() const {
     /// @todo Use direct "centrality" property if using HepMC3
     return genEvent()->heavy_ion() ? genEvent()->heavy_ion()->impact_parameter() : -1;
   }
 
   ParticlePair Event::beams() const { return Rivet::beams(*this); }
 
   double Event::sqrtS() const { return Rivet::sqrtS(beams()); }
 
   double Event::asqrtS() const { return Rivet::asqrtS(beams()); }
 
 
 
   void Event::_init(const GenEvent& ge) {
     // Use Rivet's preferred units if possible
     #ifdef HEPMC_HAS_UNITS
     _genevent.use_units(HepMC::Units::GEV, HepMC::Units::MM);
     #endif
   }
 
 
   const Particles& Event::allParticles() const {
     if (_particles.empty()) { //< assume that empty means no attempt yet made
       for (const GenParticle* gp : particles(genEvent())) {
         _particles += Particle(gp);
       }
     }
     return _particles;
   }
 
 
 }