diff --git a/src/cut/sdpcuts/CouenneMatrix.hpp b/src/cut/sdpcuts/CouenneMatrix.hpp
index 38e1b8e..11e5365 100644
--- a/src/cut/sdpcuts/CouenneMatrix.hpp
+++ b/src/cut/sdpcuts/CouenneMatrix.hpp
@@ -69,7 +69,7 @@ namespace Couenne {
 
     struct compare_scalars {
       inline bool operator() (register CouenneScalar * const &a, 
-			      register CouenneScalar * const &b)
+			      register CouenneScalar * const &b) const
       {return a -> getIndex () < b -> getIndex ();}
     };