diff --git a/functionspace/ReferenceSpace.cpp b/functionspace/ReferenceSpace.cpp
index 1c4b20dfbd3f445ab6675ce29ed2e13ccdb9293d..4d55eb875b3c889f39dcc99a924aef399a33fe09 100644
--- a/functionspace/ReferenceSpace.cpp
+++ b/functionspace/ReferenceSpace.cpp
@@ -253,8 +253,8 @@ bool ReferenceSpace::isSameEdge(const std::vector<size_t>& pTest,
   return match;
 }
 
-bool ReferenceSpace::EdgeComparator::operator()(const std::vector<size_t>& a,
-                                                const std::vector<size_t>& b){
+bool ReferenceSpace::EdgeComparator::operator()(const vector<size_t>& a,
+                                                const vector<size_t>& b) const{
   size_t maxA, minA;
   size_t maxB, minB;
 
diff --git a/functionspace/ReferenceSpace.h b/functionspace/ReferenceSpace.h
index bcd069d291579032f603a688e3598564a0326f82..3728b7354b500bdeaffeb8de76da5d37be4db8de 100644
--- a/functionspace/ReferenceSpace.h
+++ b/functionspace/ReferenceSpace.h
@@ -66,7 +66,7 @@ namespace sf{
     class EdgeComparator{
     public:
       bool operator()(const std::vector<size_t>& a,
-                      const std::vector<size_t>& b);
+                      const std::vector<size_t>& b) const;
     };
 
   private: