diff --git a/lib/process.js b/lib/process.js
index f8b5ede8..5cc11de1 100644
--- a/lib/process.js
+++ b/lib/process.js
@@ -144,6 +144,6 @@ module.exports = function(src, filename, config) {
 
   return {
     code: output.code,
-    map
+    map: map && map.toJSON()
   }
 }