diff --git a/lib/6to5/transformation/file.js b/lib/6to5/transformation/file.js
index 75a1cb0ddc..c23360d713 100644
--- a/lib/6to5/transformation/file.js
+++ b/lib/6to5/transformation/file.js
@@ -420,6 +420,7 @@ File.prototype.generate = function () {
 
   if (opts.sourceMap === "inline") {
     result.code += "\n" + util.sourceMapToComment(result.map);
+    result.map = null;
   }
 
   return result;