diff --git a/src/averagedistanceassembler.hh b/src/averagedistanceassembler.hh
index 6881c9facc619511c6a63b8cf075ac6e248602c1..0aa71a2578fbead715d8d83b3bfd666c9ac7b217 100644
--- a/src/averagedistanceassembler.hh
+++ b/src/averagedistanceassembler.hh
@@ -18,7 +18,7 @@ public:
           weights_(weights)
     {}
 
-    double value(const TargetSpace& x) {
+    double value(const TargetSpace& x) const {
 
         double result = 0;
         for (size_t i=0; i<coefficients_.size(); i++) {