diff --git a/lib/css-transform.js b/lib/css-transform.js
index 7f6ea14..b2b3126 100644
--- a/lib/css-transform.js
+++ b/lib/css-transform.js
@@ -53,7 +53,7 @@ module.exports = function ({ config, entry, minify, outfile, watch }) {
           mkdirp.sync(path.dirname(outfile))
           fs.writeFileSync(outfile, results.css)
           if (results.map) {
-            fs.writeFileSync(`${outfile}.map`, results.map)
+            fs.writeFileSync(`${outfile}.map`, results.map.toString())
           }
           if (watch) {
             logger.log(`updated ${outfile}`)