diff --git a/lemon/hartmann_orlin.h b/lemon/hartmann_orlin.h
--- a/lemon/hartmann_orlin.h
+++ b/lemon/hartmann_orlin.h
@@ -241,6 +241,10 @@
 
     /// @}
 
+  protected:
+
+    HartmannOrlin() {}
+
   public:
 
     /// \brief Constructor.