diff --git a/include/mapnik/transform/transform_expression.hpp b/include/mapnik/transform/transform_expression.hpp
index 1b6649f42..18c5ac092 100644
--- a/include/mapnik/transform/transform_expression.hpp
+++ b/include/mapnik/transform/transform_expression.hpp
@@ -171,7 +171,7 @@ struct transform_node
         : base_() {}
 
     template <typename T>
-    transform_node(T const& val)
+    explicit transform_node(T const& val)
         : base_(val) {}
 
     template <typename T>