diff --git a/lib/6to5/polyfill.js b/lib/6to5/polyfill.js
index 248d028a58..f9a0420916 100644
--- a/lib/6to5/polyfill.js
+++ b/lib/6to5/polyfill.js
@@ -1,4 +1,9 @@
 "use strict";
 
+if (global._6to5Polyfill) {
+  throw new Error("only one instance of 6to5/polyfill is allowed");
+}
+global._6to5Polyfill = true;
+
 require("core-js/shim");
 require("regenerator-6to5/runtime");