Need help on enableEtagSupport - etag not showing

I got the issue when setting enableEtagSupport to be true but no etag included in the response header
What else do I need to do 

below are my settings

`    cache: {
      enabled: true,
      type: 'mem',
      enableEtagSupport: true,
      enableXCacheHeaders: true,
     maxAge: CACHE_AGE_MEDIUM, models: [...], ...}`