[ruby-on-rails] invalid multibyte char (US-ASCII) with Rails and Ruby 1.9

I just want to add my solution:

I use german umlauts like ö, ü, ä and got the same error.
@Jarek Zmudzinski just told you how it works, but here is mine:

Add this code to the top of your Controller: # encoding: UTF-8
(for example to use flash message with umlauts)

example of my Controller:

# encoding: UTF-8
class UserController < ApplicationController

Now you can use ö, ä ,ü, ß, "", etc.