diff --git a/Framework/DataObjects/src/EventList.cpp b/Framework/DataObjects/src/EventList.cpp
index 1fc734553605675a97236707d33200a5260b4092..d3ec753e37850f3c731d75fc879b14b07e8323e3 100644
--- a/Framework/DataObjects/src/EventList.cpp
+++ b/Framework/DataObjects/src/EventList.cpp
@@ -1149,7 +1149,7 @@ void EventList::sortPulseTimeTOFDelta(const Types::Core::DateAndTime &start,
   // Avoid sorting from multiple threads
   std::lock_guard<std::mutex> _lock(m_sortMutex);
 
-  std::function<bool(TofEvent &, TofEvent &)> comparator =
+  std::function<bool(const TofEvent &, const TofEvent &)> comparator =
       comparePulseTimeTOFDelta(start, seconds);
 
   switch (eventType) {