diff --git a/include/HEJ/EventReader.hh b/include/HEJ/EventReader.hh
index a9c623c..43c861f 100644
--- a/include/HEJ/EventReader.hh
+++ b/include/HEJ/EventReader.hh
@@ -1,55 +1,55 @@
 /** \file
  *  \brief Header file for event reader interface
  *
  *  This header defines an abstract base class for reading events from files.
  *
  *  \authors   The HEJ collaboration (see AUTHORS for details)
  *  \date      2019-2022
  *  \copyright GPLv2 or later
  */
 #pragma once
 
 #include <cstddef>
 #include <memory>
 #include <optional>
 #include <string>
 
 namespace LHEF {
   class HEPEUP;
   class HEPRUP;
 }
 
 namespace HEJ {
 
   //! Abstract base class for reading events from files
   struct EventReader {
     //! Read an event
     virtual bool read_event() = 0;
 
     //! Access header text
     virtual std::string const & header() const = 0;
 
     //! Access run information
     virtual LHEF::HEPRUP const & heprup() const = 0;
 
     //! Access last read event
     virtual LHEF::HEPEUP const & hepeup() const = 0;
 
     //! Guess number of events from header
     virtual std::optional<std::size_t> number_events() const = 0;
 
-    virtual double scalefactor() {
+    virtual double scalefactor() const {
       return 1.;
     };
 
     virtual ~EventReader() = default;
   };
 
   //! Factory function for event readers
   /**
    *  @param filename   The name of the input file
    *  @returns          A pointer to an instance of an EventReader
    *                    for the input file
    */
   std::unique_ptr<EventReader> make_reader(std::string const & filename);
 } // namespace HEJ
diff --git a/include/HEJ/LesHouchesReader.hh b/include/HEJ/LesHouchesReader.hh
index 087ac14..9f7cc31 100644
--- a/include/HEJ/LesHouchesReader.hh
+++ b/include/HEJ/LesHouchesReader.hh
@@ -1,88 +1,88 @@
 /** \file
  *  \brief Header file for reading events in the Les Houches Event File format.
  *
  *  \authors   The HEJ collaboration (see AUTHORS for details)
  *  \date      2019-2022
  *  \copyright GPLv2 or later
  */
 #pragma once
 
 #include <string>
 #include <numeric>
 
 #include "LHEF/LHEF.h"
 
 #include "HEJ/EventReader.hh"
 #include "HEJ/stream.hh"
 
 namespace HEJ {
 
   //! Class for reading events from a file in the Les Houches Event File format
   class LesHouchesReader : public EventReader{
   public:
     //! Contruct object reading from the given file
     explicit LesHouchesReader(std::string const & filename);
 
     //! Read an event
     bool read_event() override;
 
     //! Access header text
     std::string const & header() const override {
       return reader_.headerBlock;
     }
 
     //! Access run information
     LHEF::HEPRUP const & heprup() const override {
       return reader_.heprup;
     }
 
     //! Access last read event
     LHEF::HEPEUP const & hepeup() const override {
       return reader_.hepeup;
     }
 
     std::optional<size_t> number_events() const override {
       std::size_t start = header().rfind("Number of Events");
       start = header().find_first_of("123456789", start);
       if(start == std::string::npos) {
         return {};
       }
       const std::size_t end = header().find_first_not_of("0123456789", start);
       return std::stoi(header().substr(start, end - start));
     }
 
-    double scalefactor() override {
+    double scalefactor() const override {
       if (generator_==Generator::Sherpa) {
         const std::size_t num_trials = std::accumulate(
           num_trials_.begin(), num_trials_.end(), 0
         );
         return 1./static_cast<double>(num_trials);
       }
       else
         return 1.;
     }
 
   private:
     enum class Generator{
       HEJ,
       HEJFOG,
       Sherpa,
       MG,
       unknown
     };
 
     bool calculate_XSECUP() {
       return calculate_XSECUP_;
     }
     Generator get_generator(
       LHEF::HEPRUP const & heprup, std::string const & header
     );
 
     istream stream_;
     LHEF::Reader reader_;
     std::vector<size_t> num_trials_;
     Generator generator_;
     bool calculate_XSECUP_;
   };
 
 } // namespace HEJ