Method 'add_user_to_mailchimp' missing after invitation

[rvgarimrj]

I´m getting the following error after invitation button is clicked:

Parameters: {"user"=>{"email"=>"abc@abc.com"}}
(0.1ms) begin transaction
User Exists (0.1ms) SELECT 1 AS one FROM "users" WHERE "users"."email" = 'abc@abc.com' LIMIT 1
SQL (2.7ms) INSERT INTO "users" ("confirmation_sent_at", "confirmation_token", "confirmed_at", "created_at", "current_sign_in_at", "current_sign_in_ip", "email", "encrypted_password", "last_sign_in_at", "last_sign_in_ip", "name", "remember_created_at", "reset_password_sent_at", "reset_password_token", "sign_in_count", "unconfirmed_email", "updated_at") VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?) ["confirmation_sent_at", nil], ["confirmation_token", nil], ["confirmed_at", nil], ["created_at", Tue, 24 Feb 2015 05:57:33 BRT -03:00], ["current_sign_in_at", nil], ["current_sign_in_ip", nil], ["email", "abc@abc.com"], ["encrypted_password", ""], ["last_sign_in_at", nil], ["last_sign_in_ip", nil], ["name", nil], ["remember_created_at", nil], ["reset_password_sent_at", nil], ["reset_password_token", nil], ["sign_in_count", 0], ["unconfirmed_email", nil], ["updated_at", Tue, 24 Feb 2015 05:57:33 BRT -03:00] rollback transaction
Completed 500 Internal Server Error in 1923ms

Gibbon::MailChimpError - Signup disabled for "abc@abc.com":
(gem) gibbon-1.1.5/lib/gibbon/api_category.rb:47:in call' (gem) gibbon-1.1.5/lib/gibbon/api_category.rb:57:inmethod_missing'
app/models/user.rb:66:in `add_user_to_mailchimp'
.....

private
def add_user_to_mailchimp
return if email.include?(ENV['ADMIN_EMAIL'])
mailchimp = Gibbon::API.new(ENV['MAILCHIMP_API_KEY'])
result = mailchimp.lists.subscribe({
:id => ENV['MAILCHIMP_LIST_ID'],
:email => {:email => self.email},
:double_optin => false,
:update_existing => true,
:send_welcome => true
})
Rails.logger.info("Subscribed #{self.email} to MailChimp") if result
end

All variables are set on application.yml and the method is correct at user.rb. What could be causing this ?
thank you