Rails gotchas: shoulda not_allow_values_for

If you're using should_not_allow_values_for and getting a failing test something in the lines of:

Failure:
test: User should not allow email to be set to "b lah". (UserTest)
    [/usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.1/lib/shoulda/assertions.rb:56:in `assert_rejects'
     /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.1/lib/shoulda/active_record/macros.rb:174:in `__bind_1239266378_671612'
     /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.1/lib/shoulda/context.rb:253:in `call'
     /usr/lib/ruby/gems/1.8/gems/thoughtbot-shoulda-2.10.1/lib/shoulda/context.rb:253:in `test: User should not allow email to be set to "b lah". ']:
Expected errors to include "is invalid" when email is set to "b lah", got errors: email is too short (minimum is 6 characters) ("b lah")email should look like an email address. ("b lah")

You can see in the above test that the email does have an error on it - but the error message is not the default. shoulda specifically checks for the error message - and if you don't have the default, then you need to pass it in thus:

should_not_allow_values_for :email, "b lah", :message => Authentication.bad_email_message