diff --git a/src/one-body-sample.cc b/src/one-body-sample.cc
index 292d84a4b43183daaba59b5addfe7db0217e2662..55dda18d421e12e2f3e32b7364460db31db170ab 100644
--- a/src/one-body-sample.cc
+++ b/src/one-body-sample.cc
@@ -386,7 +386,7 @@ int main(int argc, char *argv[]) {
             .assemble(localStressAssembler, vonMisesStress, true);
 
         Dune::MyVTKWriter<GridView> writer(leafView);
-        std::string filename((boost::format("obs%d") % run).str());
+        std::string const filename((boost::format("obs%d") % run).str());
 
         auto const displacement_ptr =
             Dune::make_shared<VTKBasisGridFunction<P1Basis, VectorType> const>(