Error compiling assets after upgrading bootstrap

I'm trying to upgrade the bootstrap version in a Ruby on Rails application. After migratinng from 4.0.0-alpha.6 to 4.1 using yarn I get this error when running rails assets:precompile

error

SassC::SyntaxError: Error: Undefined variable: "$alert-warning-bg".
        on line 32:20 of app/assets/stylesheets/variables/_helpers.scss
        from line 47:9 of app/assets/stylesheets/variables.scss
        from line 9:9 of app/assets/stylesheets/application.scss
        from line 1:9 of app/assets/stylesheets/admin.scss
>>   outline: lighten($alert-warning-bg, 9%);

helper.scss

.highlight {
  outline: lighten($alert-warning-bg, 9%);
}

$alert-warning-bg was removed on 4.1 and replaced with theme-color-level($color, $alert-bg-level) mixin (to make it DRY?). usage example:

theme-color-level('warning', $alert-bg-level)

or specifically on your case:

lighten(theme-color-level('warning', $alert-bg-level), 9%)