diff --git a/bin/6to5/file.js b/bin/6to5/file.js
index a06bf304ea..aeaf4001eb 100644
--- a/bin/6to5/file.js
+++ b/bin/6to5/file.js
@@ -28,7 +28,7 @@ module.exports = function (commander, filenames) {
         map.setSourceContent(filename, result.actual);
 
         consumer.eachMapping(function (mapping) {
-          map._mappings.push({
+          map._mappings.add({
             generatedLine: mapping.generatedLine + offset,
             generatedColumn: mapping.generatedColumn,
             originalLine: mapping.originalLine,