diff --git a/lib/carto/tree/definition.js b/lib/carto/tree/definition.js
index bfe4fd0..cf2eb28 100644
--- a/lib/carto/tree/definition.js
+++ b/lib/carto/tree/definition.js
@@ -129,7 +129,7 @@ tree.Definition.prototype.symbolizersToXML = function(env, symbolizers, zoom) {
         if (selfclosing) {
             xml += '/>\n';
         } else {
-            if (tagcontent.indexOf('<Format') != -1) {
+            if (tagcontent && tagcontent.indexOf('<') != -1) {
                 xml += '>' + tagcontent + '</' + name + '>\n';
             } else {
                 xml += '><![CDATA[' + tagcontent + ']]></' + name + '>\n';