diff --git a/lib/Function.cpp b/lib/Function.cpp
index b4971ba..974a707 100644
--- a/lib/Function.cpp
+++ b/lib/Function.cpp
@@ -26,8 +26,6 @@ using namespace Latan;
 /******************************************************************************
  *                        DoubleFunction implementation                       *
  ******************************************************************************/
-const DoubleFunction::vecFunc DoubleFunction::nullFunction_ = nullptr;
-
 // constructor /////////////////////////////////////////////////////////////////
 DoubleFunction::DoubleFunction(const Index nArg, const vecFunc &f)
 : buffer_(new DVec)
diff --git a/lib/Function.hpp b/lib/Function.hpp
index 578865c..b3555f0 100644
--- a/lib/Function.hpp
+++ b/lib/Function.hpp
@@ -39,7 +39,7 @@ private:
 public:
     // constructor
     explicit DoubleFunction(const Index nArg = 0,
-                            const vecFunc &f = nullFunction_);
+                            const vecFunc &f = nullptr);
     // destructor
     virtual ~DoubleFunction(void) = default;
     // access
@@ -59,7 +59,6 @@ private:
 private:
     std::shared_ptr<DVec> buffer_{nullptr};
     vecFunc               f_;
-    static const vecFunc  nullFunction_;
 };
 
 template <typename... Ts>
diff --git a/lib/Model.hpp b/lib/Model.hpp
index 637a493..98bc1a8 100644
--- a/lib/Model.hpp
+++ b/lib/Model.hpp
@@ -38,14 +38,14 @@ private:
 public:
     // constructor
     DoubleModel(const Index nArg = 0, const Index nPar = 0,
-                const vecFunc &f = nullFunction_);
+                const vecFunc &f = nullptr);
     // destructor
     virtual ~DoubleModel(void) = default;
     // access
     virtual Index getNArg(void) const;
     virtual Index getNPar(void) const;
     void  setFunction(const Index nArg = 0, const Index nPar = 0,
-                      const vecFunc &f = nullFunction_);
+                      const vecFunc &f = nullptr);
     // function call
     double operator()(const DVec &data, const DVec &par) const;
     double operator()(const std::vector<double> &data,
@@ -59,7 +59,6 @@ private:
 private:
     std::shared_ptr<ModelSize> size_;
     vecFunc                    f_;
-    static const vecFunc       nullFunction_;
 };
 
 END_NAMESPACE