Expression::evaluate: Avoid casting invaild values into objects
Expression::evaluate: Avoid casting invaild values into objects

mapnik/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik