diff --git a/include/HEJ/EventReader.hh b/include/HEJ/EventReader.hh
index f92f26a..a9c623c 100644
--- a/include/HEJ/EventReader.hh
+++ b/include/HEJ/EventReader.hh
@@ -1,63 +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 {
-      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));
-    }
+    virtual std::optional<std::size_t> number_events() const = 0;
 
     virtual double scalefactor() {
       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