AR association tests not passing

models, associations and tests

class User < ActiveRecord::Base
  has_many :lead_offers
  has_many :leads, through: :lead_offers
end

class Lead < ActiveRecord::Base
  has_many :lead_offers, dependent: :destroy
  has_many :users, :through => :lead_offer
end

class LeadOffer < ActiveRecord::Base
  belongs_to :lead
  belongs_to :user
end

class LeadTest < ActiveSupport::TestCase
  should have_many(:lead_offers)
  should have_many(:users)
end

class LeadOfferTest < ActiveSupport::TestCase
  should belong_to(:lead)
  should belong_to(:user)
end


test results...
Error:
LeadTest#test_: Lead should have many users. :
NoMethodError: undefined method `klass' for nil:NilClass

NOTES:
I found an issue on the web where someone got this error and the person said they missing an association:

"You're saying that Subject.has_many :photos, through: :subject_photo_assignments, but you don't have a :subject_photo_assignments association."

I've looked at this for a while and am not seeing what is missing...

@aamax:
https://gist.github.com/aamax/62b6125a7fa4760b313ce5e033b5fe0f#file-models-associations-and-tests-L8
Line 8 should be has_many :users, :through => :lead_offers, note the pluralization