Rails invalid byte sequence in UTF-8 (ArgumentError) -

all doing rails s code downloaded server, running without problem, stubborn go past error

rails s => booting thin => rails 4.0.2 application starting in development on http://0.0.0.0:3000 => run `rails server -h` more startup options => ctrl-c shutdown server exiting /usr/share/rvm/gems/ruby-2.1.4/gems/rack-1.5.2/lib/rack/builder.rb:36:in `[]': invalid byte sequence in utf-8 (argumenterror)     /usr/share/rvm/gems/ruby-2.1.4/gems/rack-1.5.2/lib/rack/builder.rb:36:in `parse_file'     /usr/share/rvm/gems/ruby-2.1.4/gems/rack-1.5.2/lib/rack/server.rb:277:in `build_app_and_options_from_config'     /usr/share/rvm/gems/ruby-2.1.4/gems/rack-1.5.2/lib/rack/server.rb:199:in `app'     /usr/share/rvm/gems/ruby-2.1.4/gems/railties-4.0.2/lib/rails/commands/server.rb:48:in `app'     /usr/share/rvm/gems/ruby-2.1.4/gems/rack-1.5.2/lib/rack/server.rb:314:in `wrapped_app'     /usr/share/rvm/gems/ruby-2.1.4/gems/railties-4.0.2/lib/rails/commands/server.rb:75:in `start'     /usr/share/rvm/gems/ruby-2.1.4/gems/railties-4.0.2/lib/rails/commands.rb:76:in `block in <top (required)>'     /usr/share/rvm/gems/ruby-2.1.4/gems/railties-4.0.2/lib/rails/commands.rb:71:in `tap'     /usr/share/rvm/gems/ruby-2.1.4/gems/railties-4.0.2/lib/rails/commands.rb:71:in `<top (required)>'     bin/rails:4:in `require'     bin/rails:4:in `<main>' 

any little life saving tip me, banging head couple of days in-numerous installations , configurations.

thanks in advance.

use gem 'rack-utf8_sanitizer' or gem "handle_invalid_percent_encoding_requests" in gemfile , run bundle.