1 files changed, 4 insertions, 1 deletions
diff --git a/src/SessionSerializer.cc b/src/SessionSerializer.cc
index 86c815d8..363cb6ff 100644
--- a/src/SessionSerializer.cc
+++ b/src/SessionSerializer.cc
@@ -143,7 +143,10 @@ namespace {
 template <typename T> class Unique {
   typedef T type;
   struct PointerCmp {
-    inline bool operator()(const type* x, const type* y) { return *x < *y; }
+    inline bool operator()(const type* x, const type* y) const
+    {
+      return *x < *y;
+    }
   };
   std::set<const type*, PointerCmp> known;