Rails 3.2 ActionMailer Demo Failing - ruby-on-rails-3

Question

I've been using rails 3 for quite a while, but I haven't had to use ActionMailer until now. I followed the guide and expected it to work out of the box. However, it appears that I've hit some error that lies deeply inside internal rails code. Unfortunately, I can't find any mention of this error, which I'm guessing must be do to configuration.
My mailer class is:
class UserMailer < ActionMailer::Base
default :from => "notifications#example.com"
def welcome_email
#url = "http://example.com/login"
mail(:to => "test#example.com", :subject => "Welcome to My Awesome Site")
end
end
When I try to use the mailer I get:
UserMailer.welcome_email
NoMethodError: undefined method `protected_instance_variables=' for ActionController::Base:Class
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_controller/metal/compatibility.rb:22:in `block in <module:Compatibility>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/concern.rb:117:in `class_eval'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/concern.rb:117:in `append_features'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_controller/base.rb:232:in `include'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_controller/base.rb:232:in `<class:Base>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_controller/base.rb:171:in `<module:ActionController>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_controller/base.rb:3:in `<top (required)>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_view/test_case.rb:11:in `<class:TestCase>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_view/test_case.rb:10:in `<module:ActionView>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/actionpack-3.2.1/lib/action_view/test_case.rb:8:in `<top (required)>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/prototype-rails-3.2.1/lib/prototype-rails/on_load_action_view.rb:17:in `<top (required)>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:251:in `require'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:251:in `block in require'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:234:in `block in load_dependency'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:639:in `new_constants_in'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:234:in `load_dependency'
... 23 levels...
from /users/user/Documents/element/app/mailers/user_mailer.rb:1:in `<top (required)>'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:469:in `load'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:469:in `block in load_file'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:639:in `new_constants_in'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:468:in `load_file'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:353:in `require_or_load'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:502:in `load_missing_constant'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:192:in `block in const_missing'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:190:in `each'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:190:in `const_missing'
from (irb):1
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.2.1/lib/rails/commands/console.rb:47:in `start'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.2.1/lib/rails/commands/console.rb:8:in `start'
from /Users/user/.rvm/gems/ruby-1.9.2-p0/gems/railties-3.2.1/lib/rails/commands.rb:41:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
I'm using rails 3.2.1. I saw some mention of protected_instance_variables= being deprecated, which could maybe explain why it isn't defined for ActionController::Base. But, that seems like it would be a rails bug, and I would imagine that ActionMailer has been well tested. Anyone have any ideas?
Thanks

Answer 1

I had experienced a similar error on starting my rails 3.2.1 application--removing the prototype-rails gem solved it for me.
Of course I happen to be using prototype, so there is now a different problem to address.