diff --git a/dune/elasticity/common/nonlinearelasticityproblem.hh b/dune/elasticity/common/nonlinearelasticityproblem.hh
index 7c5e864fbe81832d2f5fbda178e2c529731126fe..921efe4fe34662fde61b4d5323342b5dd0791f40 100644
--- a/dune/elasticity/common/nonlinearelasticityproblem.hh
+++ b/dune/elasticity/common/nonlinearelasticityproblem.hh
@@ -16,7 +16,8 @@ public:
     typedef typename VectorType::field_type field_type;
     typedef std::shared_ptr<BasisGridFunction<Basis,VectorType> > GridFunctionPtr;
 
-                         VectorType& extForces):
+    NonlinearElasticityProblem(MaterialType& material,
+                         const VectorType& extForces):
         material_(Dune::stackobject_to_shared_ptr(material)),
         extForces_(Dune::stackobject_to_shared_ptr(extForces))
     {}
@@ -51,7 +52,7 @@ private:
     //! The involved materials
     std::shared_ptr<MaterialType> material_;
     //! The external forces
-    std::shared_ptr<VectorType> extForces_;
+    std::shared_ptr<const VectorType> extForces_;
 };
 
 #include "nonlinearelasticityproblem.cc"