diff --git a/include/mapnik/params.hpp b/include/mapnik/params.hpp
index 97f4fd4f6..397e6f986 100644
--- a/include/mapnik/params.hpp
+++ b/include/mapnik/params.hpp
@@ -23,19 +23,22 @@
 #ifndef MAPNIK_PARAMS_HPP
 #define MAPNIK_PARAMS_HPP
 
-// mapnik
+// boost
 #include <boost/variant.hpp>
 #include <boost/optional.hpp>
 #include <boost/none.hpp>
 #include <boost/lexical_cast.hpp>
 
+// mapnik
+#include <mapnik/value.hpp>
+
 // stl
 #include <string>
 #include <map>
 
 namespace mapnik
 {
-typedef boost::variant<int,double,std::string> value_holder;
+typedef boost::variant<value_null,int,double,std::string> value_holder;
 typedef std::pair<const std::string, value_holder> parameter;
 typedef std::map<const std::string, value_holder> param_map;