diff --git a/lib/6to5/transform.js b/lib/6to5/transform.js
index 0a3442dde6..366ed2ad3b 100644
--- a/lib/6to5/transform.js
+++ b/lib/6to5/transform.js
@@ -71,7 +71,7 @@ transform.test = function (task, assert) {
 
 transform._ensureTransformerNames = function (type, keys) {
   _.each(keys, function (key) {
-    if (key[0] === "_" || !_.has(transform.transformers, key)) {
+    if (!_.has(transform.transformers, key)) {
       throw new ReferenceError("unknown transformer " + key + " specified in " + type);
     }
   });