asset pipeline - Rails: "rake aborted! Sass::SyntaxError: File to import not found or unreadable" -

i'm using rails 4.2.

in rails project directory, have frontend directory:

/railsproject/frontend /railsproject/frontend/styles /railsproject/frontend/styles/main.scss 

i have added config.assets.paths << rails.root.join("frontend","styles") application.rb.

in /railsproject/app/assets/stylesheets/application.css.scss:

@import "main.scss"; 

everything works fine in development mode.

but when try precompile production, fails:

$ rails_env=production bundle exec rake assets:precompile --trace ** invoke assets:precompile (first_time) ** invoke assets:environment (first_time) ** execute assets:environment ** invoke environment (first_time) ** execute environment ** execute assets:precompile rake aborted! sass::syntaxerror: file import not found or unreadable: main.scss. (sass):17 /users/max/.rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/sass-3.4.15/lib/sass/tree/import_node.rb:67:in `rescue in import' /users/max/.rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/sass-3.4.15/lib/sass/tree/import_node.rb:45:in `import' /users/max/.rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/sass-3.4.15/lib/sass/tree/import_node.rb:28:in `imported_file' /users/max/.rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/sass-3.4.15/lib/sass/tree/import_node.rb:37:in `css_import?' etc...