EmberJS Getting Broccoli error when Ember build prod

The error I get is,says can not read property '0' of null but not sure what it refers to..

$ ember build --prod
cleaning up...
Build failed.
File: assets/api.css
The Broccoli Plugin: [broccoli-persistent-filter:CleanCSSFilter] failed with:
TypeError: Cannot read property '0' of null
    at findNameRoot (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/selectors/extractor.js:66:59)
    at extract (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/selectors/extractor.js:36:9)
    at restructure (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/selectors/restructure.js:297:22)
    at optimize (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/selectors/advanced.js:71:5)
    at minify (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/clean.js:228:5)
    at whenSourceMapReady (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/clean.js:135:7)
    at /Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/clean.js:155:14
    at CleanCssPromise.CleanCSS.minify (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css/lib/clean.js:127:42)
    at promisify (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css-promise/index.js:31:9)
    at CleanCssPromise.minify (/Users/hodaraadam/dev/Apps/api/node_modules/clean-css-promise/index.js:30:14)