diff --git a/lib/index.js b/lib/index.js
index <HASH>..<HASH> 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -173,7 +173,7 @@ module.exports = class Rucksack {
         cb && bundling.on("end", cb);
     }
 
-    bundleCSS (output) {
+    bundleCSS (output, cb) {
         output = output || this.opts.cssOutput;
         if (typeof output === "string") {
             output = new streamp.writable(output);
@@ -199,20 +199,22 @@ module.exports = class Rucksack {
             cssBundler = cssBundler.use(c);
         });
 
-        cssBundler.process(cssImports).then(result => {
+        return cssBundler.process(cssImports).then(result => {
             output.write(result.css);
             if (output !== process.stdout) {
                 output.end();
             }
+            cb();
         }).catch(e => {
             logger.log(e, "error");
+            cb(e);
         });
     }
 
     bundle () {
         return new Promise(res => {
             this.bundleJS(null, () => {
-                this.bundleCSS(() => {
+                this.bundleCSS(null, () => {
                     res();
                 });
             });
Add callback for the bundleCSS method