diff --git a/include/mapnik/image_util_jpeg.hpp b/include/mapnik/image_util_jpeg.hpp
index f0ba502ff..449130561 100644
--- a/include/mapnik/image_util_jpeg.hpp
+++ b/include/mapnik/image_util_jpeg.hpp
@@ -29,7 +29,7 @@
 
 namespace mapnik {
 namespace detail {
-int parse_jpeg_quality(std::string params);
+int parse_jpeg_quality(std::string const& params);
 }
 struct jpeg_saver
 {
diff --git a/src/image_util_jpeg.cpp b/src/image_util_jpeg.cpp
index 9a1e726a4..13860fd4e 100644
--- a/src/image_util_jpeg.cpp
+++ b/src/image_util_jpeg.cpp
@@ -43,7 +43,7 @@ jpeg_saver::jpeg_saver(std::ostream & stream, std::string const& t)
 
 namespace detail {
 
-int parse_jpeg_quality(std::string params)
+int parse_jpeg_quality(std::string const& params)
 {
     int quality = 85;
     if (params != "jpeg")