diff --git a/lib/carto/tree/definition.js b/lib/carto/tree/definition.js
index e190d9f..3a443ab 100644
--- a/lib/carto/tree/definition.js
+++ b/lib/carto/tree/definition.js
@@ -108,7 +108,7 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         xml += '    <' + name + ' ';
         for (var key in attributes) {
             var x = tree.Reference.selector(attributes[key].name);
-            if (x.serialization && x.serialization === 'content') {
+            if (x && x.serialization && x.serialization === 'content') {
                 selfclosing = false;
                 tagcontent = attributes[key].eval(env).toXML(env, true);
             } else {