diff --git a/include/mapnik/raster.hpp b/include/mapnik/raster.hpp
index 9d2e7ce62..b4a7c7f4a 100644
--- a/include/mapnik/raster.hpp
+++ b/include/mapnik/raster.hpp
@@ -31,7 +31,7 @@ namespace mapnik {
     {
         Envelope<double> ext_;
         ImageData32 data_;
-        raster(Envelope<double> const ext,ImageData32& data)
+        raster(Envelope<double> const& ext,ImageData32& data)
             : ext_(ext),
               data_(data) {}
     };