diff --git a/lib/6to5/traverse/index.js b/lib/6to5/traverse/index.js
index 1995a5b992..86ee88f0e3 100644
--- a/lib/6to5/traverse/index.js
+++ b/lib/6to5/traverse/index.js
@@ -18,7 +18,7 @@ function traverse(parent, callbacks, opts) {
 
   // unknown node type to traverse
   var keys = t.VISITOR_KEYS[parent.type];
-  if (!keys) return;
+  if (!keys) throw new Error("unknown node " + parent.type);
 
   opts = opts || {};
   if (_.isArray(opts)) opts = { blacklist: opts };
@@ -29,25 +29,32 @@ function traverse(parent, callbacks, opts) {
   // normalise callbacks
   if (_.isFunction(callbacks)) callbacks = { enter: callbacks };
 
-  _.each(keys, function (key) {
+  for (var i in keys) {
+    var key = keys[i];
     var nodes = parent[key];
-    if (!nodes) return;
+    if (!nodes) continue;
+
+    var updated = false;
 
     var handle = function (obj, key) {
       var node = obj[key];
       if (!node) return;
 
       // type is blacklisted
-      if (_.contains(blacklistTypes, node.type)) return;
+      if (blacklistTypes.indexOf(node.type) > -1) return;
 
       // replace node
       var maybeReplace = function (result) {
         if (result === false) return;
-        if (result != null) node = obj[key] = result;
+
+        if (result != null) {
+          updated = true;
+          node = obj[key] = result;
+        }
       };
 
       //
-      var opts2 = _.clone(opts);
+      var opts2 = { scope: opts.scope, blacklist: opts.blacklist };
       if (t.isScope(node)) opts2.scope = new Scope(opts.scope, node);
 
       // enter
@@ -69,15 +76,15 @@ function traverse(parent, callbacks, opts) {
     };
 
     if (_.isArray(nodes)) {
-      _.each(nodes, function (node, i) {
+      for (var i in nodes) {
         handle(nodes, i);
-      });
+      }
 
-      parent[key] = _.flatten(parent[key]);
+      if (updated) parent[key] = _.flatten(parent[key]);
     } else {
       handle(parent, key);
     }
-  });
+  }
 }
 
 traverse.removeProperties = function (tree) {