`//= require` is not working in connect-assets

I am using connect-assets on an express.js app running on development mode.

• I have app.use(require('connect-assets')()) inside app.config()
• I have main.js and util.js in <PROJECT-ROOT>/assets/js folder
• In main.js, there is only one line: //= require ./util
• In template, there is != js('main')

So, the he generated html includes <script src='/js/util.js'></script><script src='/js/main.js'></script>. However, the request to /js/main.js succeed without errors but the request to /js/util.js failed.

Am I missing anything?