Gettext_i18n_rails causing namespaced helpers to not be found

When I install gettext_i18n_rails and put the following code in my ApplicationController, I start getting errors on every page telling me that my helpers in an Admin namespace don’t exist, even though they definitely do…

FastGettext.text_domain = 'app'
FastGettext.available_locales = ['cs', 'en']
FastGettext.locale = 'cs'

e.g. “Couldn’t find Admin::BillingHelper, expected it to be defined in helpers/admin/billing_helper.rb”, the file does exist and has this in it

module Admin::BillingHelper

When I delete the helper, it just moves onto the next one with the same error. This happens even on pages that have nothing to do with the Admin namespace.

Any ideas what could be causing this?