diff --git a/lib/carto/renderer_js.js b/lib/carto/renderer_js.js
index ac4fea5..b4c2b1f 100644
--- a/lib/carto/renderer_js.js
+++ b/lib/carto/renderer_js.js
@@ -239,10 +239,11 @@ CartoCSS.prototype = {
       for(var i = 0; i < defs.length; ++i) {
         var def = defs[i];
 
-        def.toXML(parse_env, {});
-
-        if (this.options.strict && parse_env.errors.message) {
-          throw new Error(parse_env.errors.message);
+        if (this.options.strict) {
+          def.toXML(parse_env, {});
+          if (parse_env.errors.message) {
+            throw new Error(parse_env.errors.message);
+          }
         }
 
         var k = defKey(def);