diff --git a/src/nicefunction.hh b/src/nicefunction.hh
index 432ca89a6d8384f7b2fd3c21f78ed36bbc1573ae..532dfa2580e7e46404615da3ae6cb10cd9c43859 100644
--- a/src/nicefunction.hh
+++ b/src/nicefunction.hh
@@ -38,9 +38,9 @@ class TrivialFunction : public NiceFunction {
 public:
   void virtual evaluate(double const& x, double& y) const { y = 0; }
 
-  double virtual leftDifferential(const double) const { return 0; }
+  double virtual leftDifferential(double) const { return 0; }
 
-  double virtual rightDifferential(const double) const { return 0; }
+  double virtual rightDifferential(double) const { return 0; }
 };
 
 // slope in [n-1,n] is n