diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 23adb84386..adbcfa9ba7 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -82,7 +82,7 @@ File.normaliseOptions = function (opts) {
   opts = clone(opts);
 
   for (var key in opts) {
-    if (File.validOptions.indexOf(key) < 0) {
+    if (key[0] !== "_" && File.validOptions.indexOf(key) < 0) {
       throw new ReferenceError("Unknown option: " + key);
     }
   }