diff --git a/extensions/demo/NavierStokesCahnHilliard_PC/src/CahnHilliardNavierStokes_.h b/extensions/demo/NavierStokesCahnHilliard_PC/src/CahnHilliardNavierStokes_.h
index 338b35704e66264ca6dc52f3b00c75e4afb090b0..0fe8c6384e6a9c028e2a87227e956a8903ad3335 100644
--- a/extensions/demo/NavierStokesCahnHilliard_PC/src/CahnHilliardNavierStokes_.h
+++ b/extensions/demo/NavierStokesCahnHilliard_PC/src/CahnHilliardNavierStokes_.h
@@ -20,6 +20,15 @@ using namespace AMDiS;
 
 
 
+class ID : public AbstractFunction<double, double>
+{
+public:
+  ID()
+    : AbstractFunction<double, double>(2)  {};
+  double operator()(const double& x) const  {
+    return x;
+  }
+};