Rails 4 Css undefined method `empty?' for 10:Sass::Script::Number

Rails 4 Css undefined method `empty?' for 10:Sass::Script::Number

I just started a Rails 4 pet project this month, and I kept running in to
a weird problem,
undefined method `empty?' for 10:Sass::Script::Number
(in
/home/mickey/workspace/punchclock/app/assets/stylesheets/punchclock_bootstrap.css.scss)
This happened whenever I modify my CSS file, I need to commit, push,
delete the folder, clone, setup then the problem would go away = . I
haven't encounter this problem working with Rails 3, I think this only
happens to my laptop, which is running with a Virtual Box installation of
Ubuntu.
Anybody encouter this too?