diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 35230e9aa6..b42aac76ee 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -29,21 +29,10 @@ File.declarations = [
 ];
 
 File.normaliseOptions = function (opts) {
-  opts = opts || {};
-
-  _.each(opts.transformers, function (transformer, i) {
-    if (!(transformer instanceof Transformer)) {
-      throw new TypeError("opts.transformers[" + i + "] isn't an instance of Transformer");
-    }
-  });
-
-  opts = _.cloneDeep(opts, function (value) {
-    return value instanceof Transformer ? value : undefined;
-  });
+  opts = _.cloneDeep(opts || {});
 
   _.defaults(opts, {
     experimental: false,
-    transformers: [],
     playground:   false,
     whitespace:   true,
     blacklist:    [],
@@ -113,7 +102,7 @@ File.prototype.getModuleFormatter = function (type) {
   }
 
   if (!ModuleFormatter) {
-    throw new ReferenceError("unknown module formatter type " + type);
+    throw new ReferenceError("Unknown module formatter type " + JSON.stringify(type));
   }
 
   return new ModuleFormatter(this);
@@ -192,25 +181,9 @@ File.prototype.transform = function (ast) {
 
   var self = this;
 
-  var runUserTransformers = function (end) {
-    _.each(self.opts.transformers, function (transformer) {
-      if (transformer.opts.end) {
-        if (!end) return;
-      } else if (end) {
-        return;
-      }
-
-      transformer.transform(self);
-    });
-  };
-
-  runUserTransformers();
-
   _.each(transform.transformers, function (transformer) {
     transformer.transform(self);
   });
-
-  runUserTransformers(true);
 };
 
 File.prototype.generate = function () {