diff --git a/python/py_structural_mechanics_model.cc b/python/py_structural_mechanics_model.cc
index 373d2dff2..07db57faa 100644
--- a/python/py_structural_mechanics_model.cc
+++ b/python/py_structural_mechanics_model.cc
@@ -1,127 +1,127 @@
 /* -------------------------------------------------------------------------- */
 #include "py_aka_array.hh"
 /* -------------------------------------------------------------------------- */
 #include "structural_mechanics_model.hh"
 /* -------------------------------------------------------------------------- */
 #include <pybind11/pybind11.h>
 /* -------------------------------------------------------------------------- */
 namespace py = pybind11;
 /* -------------------------------------------------------------------------- */
 
 namespace akantu {
 
 /* -------------------------------------------------------------------------- */
 #define def_deprecated(func_name, mesg)                                        \
   def(func_name, [](py::args, py::kwargs) { AKANTU_ERROR(mesg); })
 
 #define def_function_nocopy(func_name)                                         \
   def(                                                                         \
       #func_name,                                                              \
       [](StructuralMechanicsModel & self) -> decltype(auto) {                  \
         return self.func_name();                                               \
       },                                                                       \
       py::return_value_policy::reference)
 
 #define def_function_(func_name)                                               \
   def(#func_name, [](StructuralMechanicsModel & self) -> decltype(auto) {      \
     return self.func_name();                                                   \
   })
 
 #define def_plainmember(M) def_readwrite(#M, &StructuralMaterial::M)
 /* -------------------------------------------------------------------------- */
 
 void register_structural_mechanics_model(pybind11::module & mod) {
   /* First we have to register the material class
    * The wrapper aims to mimic the behaviour of the real material.
    */
   py::class_<StructuralMaterial>(mod, "StructuralMaterial")
       .def(py::init<>())
       .def_plainmember(E)
       .def_plainmember(A)
       .def_plainmember(I)
       .def_plainmember(Iz)
       .def_plainmember(Iy)
       .def_plainmember(GJ)
       .def_plainmember(rho)
       .def_plainmember(t)
       .def_plainmember(nu);
 
   /* Now we create the structural model wrapper
    * Note that this is basically a port from the solid mechanic part.
    */
   py::class_<StructuralMechanicsModel, Model>(mod, "StructuralMechanicsModel")
       .def(py::init<Mesh &, UInt, const ID &, const MemoryID &>(),
            py::arg("mesh"), py::arg("spatial_dimension") = _all_dimensions,
            py::arg("id") = "structural_mechanics_model",
            py::arg("memory_id") = 0)
       .def(
           "initFull",
           [](StructuralMechanicsModel & self,
              const AnalysisMethod & analysis_method) -> void {
             self.initFull(_analysis_method = analysis_method);
           },
           py::arg("_analysis_method"))
       .def("initFull",
            [](StructuralMechanicsModel & self) -> void { self.initFull(); })
       .def_function_nocopy(getExternalForce)
       .def_function_nocopy(getDisplacement)
       .def_function_nocopy(getInternalForce)
       .def_function_nocopy(getVelocity)
       .def_function_nocopy(getAcceleration)
       .def_function_nocopy(getInternalForce)
       .def_function_nocopy(getBlockedDOFs)
       .def_function_nocopy(getMesh)
 
       /*
        * These functions are basically untested.
        */
       .def(
           "getElementMaterialMap",
           [](StructuralMechanicsModel & self, const ElementType & type,
-             GhostType ghost_type) {
+             GhostType ghost_type) -> decltype(auto) {
             return self.getElementMaterial(type, ghost_type);
           },
           "This function returns the map that maps elements to materials.",
           py::arg("type"), py::arg("ghost_type") = _not_ghost,
           py::return_value_policy::reference)
       .def(
           "getMaterialOf",
           [](StructuralMechanicsModel & self, Element element) {
             return self.getMaterial(element);
           },
           "This function returns the `StructuralMaterial` instance that is "
           "associated with element `element`."
           " It is important that the returned object can be modified, but this "
           "will not affect the material stored inside the model."
           " If you want to change the material, use `addMaterial()` to add a "
           "new one and then manipulate the mapping by operating on "
           "`getElementMaterialMap()`.",
           py::arg("element"),
           py::return_value_policy::copy // By using the copy operation, we
                                         // completly decouple the C++ and Python
                                         // part.
           )
       .def(
           "addMaterial",
           [](StructuralMechanicsModel & self, StructuralMaterial & mat)
               -> UInt { return self.addMaterial(mat); },
           "This function adds the `StructuralMaterial` `mat` to `self`."
           " The function returns the ID of the new material.",
           py::arg("mat"))
       .def(
           "getMaterialByID",
           [](StructuralMechanicsModel & self, UInt i) {
             return self.getMaterialByID(i);
           },
           "This function returns the `i`th material of `self`", py::arg("i"),
           py::return_value_policy::copy // Everything will be coupled so a
                                         // complet decoupling
           )
       .def(
           "getNbMaterials",
           [](StructuralMechanicsModel & self) { return self.getNbMaterials(); },
           "Returns the number of different materials inside `self`."
           " The highest ID is one less than the returned number.");
 } // End: register structural mechanical model
 
 } // namespace akantu