Mailer class error

I created a helper method in a file called mailer_helper.rb that I
want to use in my mailer class. I included the helper using this
statement:
helper :mailer
even after including this I get this error:
undefined local variable or method `get_host' for #<Mailer:0x38aca0c>
any ideas?

class Mailer < ActionMailer::Base

  helper :mailer

  def contact_us(contact_us)
    @subject = 'Nu+riSum contact request'
    @recipients = 'me@me.com'
    @from = contact_us.email
    @sent_on = Time.now
    @layout = :mailer
    @body =
    {
      :fn => contact_us.first_name,
      :ln => contact_us.last_name,
      :em => contact_us.email,
      :org => contact_us.organization,
      :ph => contact_us.phone,
      :msg => contact_us.comments,
      :host => get_host
    }
  end