diff --git a/lib/transform.js b/lib/transform.js
index 2ba004ee..976dd058 100644
--- a/lib/transform.js
+++ b/lib/transform.js
@@ -59,7 +59,7 @@ TreeTransformer.prototype = new TreeWalker;
         node.DEFMETHOD("transform", function(tw, in_list){
             var x, y;
             tw.push(this);
-            x = tw.before(this, descend, in_list);
+            if (tw.before) x = tw.before(this, descend, in_list);
             if (x === undefined) {
                 if (!tw.after) {
                     x = this;