NoMethodError: undefined method `devise_mail' for MyMailer:Class

Question

I've got a very simple custom mailer that I am using with Devise, however it is unable to find the helper method "devise_mail".

If I clone the devise_mail method within the class (Uncommenting in the sample below) the mailer works with the custom instructions as I'd expect so the custom mailer is definately been hit, and the custom subject line used elsewhere also works with devise invitable.

The full error is:

NoMethodError: undefined method devise_mail' for MyMailer:Class from /Users/paul/.rvm/gems/ruby-2.0.0-p598/gems/attr_encrypted-1.3.4/lib/attr_encrypted.rb:259:inmethod_missing'

What am I missing?

class MyMailer < Devise::Mailer
  protected
  helper :application
  include Devise::Mailers::Helpers
  include DeviseInvitable::Mailer
  default template_path: 'devise/mailer'

  def subject_for(key)
    return super  unless key.to_s == 'invitation_instructions'

    I18n.t('devise.mailer.invitation_instructions.subject', 
      :invited_by => resource.invited_by.try(:firstName) || 'Someone')
  end

  def self.custom_invitation_instructions(record, opts={})
    devise_mail(record, :custom_invitation_instructions, opts)
  end
# Clone of the helper method within this class
#  def devise_mail(record, action, opts={}) 
#    initialize_from_record(record)
#    mail headers_for(action, opts)
#  end
end

Thanks in advance for any thoughts.