diff --git a/dumux/discretization/box/fvgridgeometry.hh b/dumux/discretization/box/fvgridgeometry.hh
index 7459d1aa9a25c3138af965de31aa64d85c008abb..aaf43e422dc5656fb897d56059796bc5e24c0469 100644
--- a/dumux/discretization/box/fvgridgeometry.hh
+++ b/dumux/discretization/box/fvgridgeometry.hh
@@ -314,7 +314,7 @@ public:
     { return periodicVertexMap_.at(dofIdx); }
 
     //! Returns the map between dofs across periodic boundaries
-    const std::unordered_map<std::size_t, std::size_t> periodicVertexMap() const
+    const std::unordered_map<std::size_t, std::size_t>& periodicVertexMap() const
     { return periodicVertexMap_; }
 
 private:
@@ -505,7 +505,7 @@ public:
     { return periodicVertexMap_.at(dofIdx); }
 
     //! Returns the map between dofs across periodic boundaries
-    const std::unordered_map<std::size_t, std::size_t> periodicVertexMap() const
+    const std::unordered_map<std::size_t, std::size_t>& periodicVertexMap() const
     { return periodicVertexMap_; }
 
 private: