diff --git a/analyses/pluginMC/MC_XS.cc b/analyses/pluginMC/MC_XS.cc
--- a/analyses/pluginMC/MC_XS.cc
+++ b/analyses/pluginMC/MC_XS.cc
@@ -1,92 +1,94 @@
 // -*- C++ -*-
 #include "Rivet/Analysis.hh"
+#include "Rivet/AnalysisHandler.hh"
 
 #ifndef RIVET_ENABLE_HEPMC_3
 #include "HepMC/HepMCDefs.h"
 #endif
 
 namespace Rivet {
 
   /// @brief Analysis for the generated cross section
   class MC_XS : public Analysis {
   public:
 
     /// @name Constructors etc.
     //@{
 
     /// Constructor
     DEFAULT_RIVET_ANALYSIS_CTOR(MC_XS);
 
     //@}
 
 
   public:
 
     /// @name Analysis methods
     //@{
 
     /// Book histograms and initialise projections before the run
     void init() {
       /// @todo Convert to Scatter1D or Counter
       book(_h_XS, "XS");
       book(_h_N, "N", 1, 0.0, 1.0);
       book(_h_pmXS, "pmXS", 2, -1.0, 1.0);
       book(_h_pmN, "pmN", 2, -1.0, 1.0);
 
       _mc_xs = _mc_error = 0.;
     }
 
     /// Perform the per-event analysis
     void analyze(const Event& event) {
 
       #if defined RIVET_ENABLE_HEPMC_3
       //@todo HepMC3::GenCrossSection methods aren't const accessible :(
       RivetHepMC::GenCrossSection gcs = *(event.genEvent()->cross_section());
       _mc_xs    = gcs.xsec();
       _mc_error = gcs.xsec_err();
       #elif defined HEPMC_HAS_CROSS_SECTION
       _mc_xs    = event.genEvent()->cross_section()->cross_section();
       _mc_error = event.genEvent()->cross_section()->cross_section_error();
       #endif // VERSION_CODE >= 3000000
 
-      for (size_t m = 0; m < event.weights().size(); ++m) {
+      assert(event.weights().size() >= handler().numWeights());
+      for (size_t m = 0; m < handler().numWeights(); ++m) {
         const double weight = event.weights()[m];
         _h_pmXS.get()->_getPersistent(m)->fill(0.5*(weight > 0 ? 1. : -1), abs(weight));
         _h_pmN.get()->_getPersistent(m)->fill(0.5*(weight > 0 ? 1. : -1), 1.);
         _h_N.get()->_getPersistent(m)->fill(0.5, 1.0);
       }
       
     }
 
 
     /// Normalise histograms etc., after the run
     void finalize() {
       scale(_h_pmXS, crossSection()/sumOfWeights());
       scale(_h_pmN, 1.0/numEvents());
       scale(_h_N, 1.0/numEvents());
       #ifndef HEPMC_HAS_CROSS_SECTION
       _mc_xs = crossSection();
       _mc_error = 0.0;
       #endif
       _h_XS->addPoint(0, _mc_xs, 0.5, _mc_error);
     }
 
     //@}
 
 
   private:
 
     /// @name Histograms
     //@{
     Scatter2DPtr _h_XS;
     Histo1DPtr _h_pmXS, _h_pmN, _h_N;
     double _mc_xs, _mc_error;
     //@}
 
   };
 
 
   // The hook for the plugin system
   DECLARE_RIVET_PLUGIN(MC_XS);
 
 }