diff --git a/src/mysolver.cc b/src/mysolver.cc
index 10f4f703c9b1335067e4a1e12a8de3dae1f4d520..d8771464461d52de84dac93d777f367757b42f15 100644
--- a/src/mysolver.cc
+++ b/src/mysolver.cc
@@ -13,7 +13,7 @@
 template <int dim, class MatrixType, class VectorType, class GridType,
           class BlockProblemType>
 MySolver<dim, MatrixType, VectorType, GridType, BlockProblemType>::MySolver(
-    Dune::ParameterTree parset, int refinements, double solver_tolerance,
+    Dune::ParameterTree const &parset, int refinements, double solver_tolerance,
     GridType const &grid, Dune::BitSetVector<dim> const &ignoreNodes)
     : baseEnergyNorm(linearBaseSolverStep),
       linearBaseSolver(&linearBaseSolverStep,
diff --git a/src/mysolver.hh b/src/mysolver.hh
index a8a5c60d0f8f3aae74eb5d36faba29f0aa0848c8..a87ed9153338918fffb3bcafdd1c878c8e9b6a9b 100644
--- a/src/mysolver.hh
+++ b/src/mysolver.hh
@@ -22,8 +22,9 @@ class MySolver {
                                             NonlinearSmootherType> SolverType;
 
 public:
-  MySolver(Dune::ParameterTree parset, int refinements, double solver_tolerance,
-           GridType const &grid, Dune::BitSetVector<dim> const &ignoreNodes);
+  MySolver(Dune::ParameterTree const &parset, int refinements,
+           double solver_tolerance, GridType const &grid,
+           Dune::BitSetVector<dim> const &ignoreNodes);
 
   ~MySolver();