diff --git a/lib/ast.js b/lib/ast.js
index a7885a79..7b4c03d7 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -160,6 +160,8 @@ var restore_transforms = [];
 AST_Node.enable_validation = function() {
     AST_Node.disable_validation();
     (function validate_transform(ctor) {
+        ctor.SUBCLASSES.forEach(validate_transform);
+        if (!HOP(ctor.prototype, "transform")) return;
         var transform = ctor.prototype.transform;
         ctor.prototype.transform = function(tw, in_list) {
             var node = transform.call(this, tw, in_list);
@@ -173,7 +175,6 @@ AST_Node.enable_validation = function() {
         restore_transforms.push(function() {
             ctor.prototype.transform = transform;
         });
-        ctor.SUBCLASSES.forEach(validate_transform);
     })(this);
 };