diff --git a/lib/Functional/Function.cpp b/lib/Functional/Function.cpp
index 6b31feb..18e286f 100644
--- a/lib/Functional/Function.cpp
+++ b/lib/Functional/Function.cpp
@@ -180,7 +180,7 @@ DoubleFunction DoubleFunction::bind(const Index argIndex,
 }
 
 // sample //////////////////////////////////////////////////////////////////////
-DVec DoubleFunction::sample(const DMat &x)
+DVec DoubleFunction::sample(const DMat &x) const
 {
     if (x.cols() != getNArg())
     {
diff --git a/lib/Functional/Function.hpp b/lib/Functional/Function.hpp
index 859c705..82bde35 100644
--- a/lib/Functional/Function.hpp
+++ b/lib/Functional/Function.hpp
@@ -57,7 +57,7 @@ public:
     DoubleFunction bind(const Index argIndex, const double val) const;
     DoubleFunction bind(const Index argIndex, const DVec &x) const;
     // sample
-    DVec sample(const DMat &x);
+    DVec sample(const DMat &x) const;
     // arithmetic operators
     DoubleFunction   operator-(void) const;
     DoubleFunction & operator+=(const DoubleFunction &f);