diff --git a/include/mapnik/json/extract_bounding_boxes_x3.hpp b/include/mapnik/json/extract_bounding_boxes_x3.hpp
index fc3cb4772..39ede302c 100644
--- a/include/mapnik/json/extract_bounding_boxes_x3.hpp
+++ b/include/mapnik/json/extract_bounding_boxes_x3.hpp
@@ -27,7 +27,7 @@
 namespace mapnik { namespace json {
 
 template <typename Iterator, typename Boxes>
-void extract_bounding_boxes(Iterator start, Iterator end, Boxes & boxes);
+void extract_bounding_boxes(Iterator & start, Iterator const& end, Boxes & boxes);
 
 }}
 
diff --git a/src/json/extract_bounding_boxes_x3.cpp b/src/json/extract_bounding_boxes_x3.cpp
index 0c7a97d71..7aadea8e5 100644
--- a/src/json/extract_bounding_boxes_x3.cpp
+++ b/src/json/extract_bounding_boxes_x3.cpp
@@ -175,7 +175,7 @@ BOOST_SPIRIT_DEFINE (
 }
 
 template <typename Iterator, typename Boxes>
-void extract_bounding_boxes(Iterator start, Iterator end, Boxes & boxes)
+void extract_bounding_boxes(Iterator& start, Iterator const& end, Boxes & boxes)
 {
     using namespace boost::spirit;
     using space_type = mapnik::json::grammar::space_type;
@@ -198,6 +198,6 @@ void extract_bounding_boxes(Iterator start, Iterator end, Boxes & boxes)
 
 }
 using base_iterator_type = char const*;
-template void extract_bounding_boxes<base_iterator_type, boxes_type>(base_iterator_type, base_iterator_type, boxes_type&);
-template void extract_bounding_boxes<base_iterator_type, boxes_type_f>(base_iterator_type, base_iterator_type, boxes_type_f&);
+template void extract_bounding_boxes<base_iterator_type, boxes_type>(base_iterator_type&, base_iterator_type const&, boxes_type&);
+template void extract_bounding_boxes<base_iterator_type, boxes_type_f>(base_iterator_type&, base_iterator_type const& , boxes_type_f&);
 }}