How to use the I18n fallback features in Rails 3

I am getting an "translation missing" error message from Rails 3:

activerecord:  notices:    messages:      success:         create: "Something was created"    models:      user:        success:          create: "Thanks for registration"I18n.t("activerecord.notices.models.user.success.create")# => "Thanks for registration"I18n.t("activerecord.notices.models.book.success.create") # => "translation missing: de, activerecord, notices, models, book, success, create"

I don't know why the book model doesn't get the fallback massage. I have set config.i18n.fallbacks = true.