validate_acceptance_of behaviour

invitation.rb

class Invitation
  extend ActiveModel::Naming
  include ActiveModel::Conversion
  include ActiveModel::Validations

  attr_accessor :terms

  validates_acceptance_of   :terms, :on => :save, :allow_nil => false

  def initialize(attrs={})
    attrs.each do |k, v| send("#{k}=", v) end
  end

end

invitation_test.rb

require 'test_helper'

class InvitedUserTest < ActiveSupport::TestCase

  test "terms validation" do
    iu = Invitation.new
    assert iu.valid? == false
    assert_nil iu.terms

    # Fails when :on => :save in model, but works when :on is omitted
    assert iu.errors.include?(:terms), "Terms MUST NOT be nil #{iu.errors.full_messages}"
  end


end

Solution found, I was referring to outdated documentation.

Huge thanks to elaptics on #rubyonrails

The solution is to omit the :on parameter.