diff --git a/dumux/discretization/box/elementvolumevariables.hh b/dumux/discretization/box/elementvolumevariables.hh
index 59c90dc1c3844d36e5bf9a761c7d54899b7b2bbf..4310baa62e4471a55a35457c0a1e68285ebf207a 100644
--- a/dumux/discretization/box/elementvolumevariables.hh
+++ b/dumux/discretization/box/elementvolumevariables.hh
@@ -24,6 +24,7 @@
 #define DUMUX_DISCRETIZATION_BOX_ELEMENT_VOLUMEVARIABLES_HH
 
 #include <dumux/implicit/properties.hh>
+#include <dumux/implicit/model.hh>
 
 namespace Dumux
 {
diff --git a/dumux/porousmediumflow/implicit/fluxvariablescache.hh b/dumux/porousmediumflow/implicit/fluxvariablescache.hh
index 6d6725745629b843350a39b5559d8b7b629b19c2..bda7181eb175f546fa0e14a1870923d1d829caea 100644
--- a/dumux/porousmediumflow/implicit/fluxvariablescache.hh
+++ b/dumux/porousmediumflow/implicit/fluxvariablescache.hh
@@ -35,6 +35,12 @@ template<class TypeTag, DiscretizationMethods Method>
 class PorousMediumFluxVariablesCacheImplementation
 {};
 
+namespace Properties
+{
+// forward declaration
+NEW_PROP_TAG(NumPhases);
+}
+
 /*!
  * \ingroup ImplicitModel
  * \brief The flux variables cache classes for porous media.