diff --git a/lib/6to5/runtime.js b/lib/6to5/runtime.js
index 39ac7ba908..6c4fdbc9fd 100644
--- a/lib/6to5/runtime.js
+++ b/lib/6to5/runtime.js
@@ -5,7 +5,7 @@ var t         = require("./types");
 var _         = require("lodash");
 
 module.exports = function (namespace) {
-  namespace = t.identifier(namespace || "to5Runtime");
+  namespace = t.identifier(t.toIdentifier(namespace || "to5Runtime"));
 
   var body      = [];
   var container = t.functionExpression(null, [], t.blockStatement(body));