diff --git a/include/mapnik/geometry.hpp b/include/mapnik/geometry.hpp
index 91147d7fa..d6b202ea0 100644
--- a/include/mapnik/geometry.hpp
+++ b/include/mapnik/geometry.hpp
@@ -51,7 +51,7 @@ namespace mapnik {
     public:
         geometry () {}	
 	
-        Envelope<double> envelope()
+        Envelope<double> envelope() const
         {
             Envelope<double> result;		
             double x,y;