diff --git a/bindings/python/mapnik_grid.cpp b/bindings/python/mapnik_grid.cpp
index 87a7ef852..4bff835c8 100644
--- a/bindings/python/mapnik_grid.cpp
+++ b/bindings/python/mapnik_grid.cpp
@@ -32,7 +32,7 @@
 using namespace boost::python;
 
 // help compiler see template definitions
-static dict (*encode)( mapnik::grid const&, std::string, bool, unsigned int) = mapnik::grid_encode;
+static dict (*encode)( mapnik::grid const&, std::string const& , bool, unsigned int) = mapnik::grid_encode;
 
 bool painted(mapnik::grid const& grid)
 {
diff --git a/bindings/python/mapnik_grid_view.cpp b/bindings/python/mapnik_grid_view.cpp
index 926ee18f6..aa29dda78 100644
--- a/bindings/python/mapnik_grid_view.cpp
+++ b/bindings/python/mapnik_grid_view.cpp
@@ -34,7 +34,7 @@
 using namespace boost::python;
 
 // help compiler see template definitions
-static dict (*encode)( mapnik::grid_view const&, std::string, bool, unsigned int) = mapnik::grid_encode;
+static dict (*encode)( mapnik::grid_view const&, std::string const& , bool, unsigned int) = mapnik::grid_encode;
 
 void export_grid_view()
 {
diff --git a/bindings/python/python_grid_utils.cpp b/bindings/python/python_grid_utils.cpp
index ab4a90b49..3312c8246 100644
--- a/bindings/python/python_grid_utils.cpp
+++ b/bindings/python/python_grid_utils.cpp
@@ -342,7 +342,7 @@ void grid_encode_utf(T const& grid_type,
 }
 
 template <typename T>
-boost::python::dict grid_encode( T const& grid, std::string format, bool add_features, unsigned int resolution)
+boost::python::dict grid_encode( T const& grid, std::string const& format, bool add_features, unsigned int resolution)
 {
     if (format == "utf") {
         boost::python::dict json;
@@ -357,8 +357,8 @@ boost::python::dict grid_encode( T const& grid, std::string format, bool add_fea
     }
 }
 
-template boost::python::dict grid_encode( mapnik::grid const& grid, std::string format, bool add_features, unsigned int resolution);
-template boost::python::dict grid_encode( mapnik::grid_view const& grid, std::string format, bool add_features, unsigned int resolution);
+template boost::python::dict grid_encode( mapnik::grid const& grid, std::string const& format, bool add_features, unsigned int resolution);
+template boost::python::dict grid_encode( mapnik::grid_view const& grid, std::string const& format, bool add_features, unsigned int resolution);
 
 /* new approach: key comes from grid object
  * grid size should be same as the map
diff --git a/bindings/python/python_grid_utils.hpp b/bindings/python/python_grid_utils.hpp
index 30ed4ff43..2c4083d1b 100644
--- a/bindings/python/python_grid_utils.hpp
+++ b/bindings/python/python_grid_utils.hpp
@@ -64,7 +64,7 @@ void grid_encode_utf(T const& grid_type,
                             unsigned int resolution);
 
 template <typename T>
-boost::python::dict grid_encode( T const& grid, std::string format, bool add_features, unsigned int resolution);
+boost::python::dict grid_encode( T const& grid, std::string const& format, bool add_features, unsigned int resolution);
 
 /* new approach: key comes from grid object
  * grid size should be same as the map