diff --git a/src/properscalarincreasingconvexfunction.hh b/src/properscalarincreasingconvexfunction.hh
index da30eb4b0f44c2ea68c929fdea7e30ba89619db0..d02ad9413bb896e59239ad0ec5babcf0be05b09f 100644
--- a/src/properscalarincreasingconvexfunction.hh
+++ b/src/properscalarincreasingconvexfunction.hh
@@ -1,3 +1,6 @@
+#ifndef PROPER_SCALAR_INCREASING_CONVEX_FUNCTION_HH
+#define PROPER_SCALAR_INCREASING_CONVEX_FUNCTION_HH
+
 class ProperScalarIncreasingConvexFunction {
 public:
   virtual double operator()(const double s) const = 0;
@@ -22,3 +25,5 @@ class TrivialFunction : public ProperScalarIncreasingConvexFunction {
 
   double rightDifferential(const double) const { return 0; }
 };
+
+#endif