No Method Error while testing with Rspec in Rails

I get a No Method Error while Testing with Rspec. Here is my code for the Test:

scenario 'admin updates a commissionrate' do
  create(:assignment, user_id: @user.id, role_id: 1)
  sign_in(@user)
  commissionrate = create(:commissionrate, rate: 0.05)

  visit edit_commissionrate_path(commissionrate)

  fill_in 'rate', with: 0.05
  click_on 'Eingaben speichern'

  expect(page).to have_content('erfolgreich')
end

and this is the factory for the commissionrate:

require 'faker'

FactoryGirl.define do
  factory :commissionrate do
    commissionrate     { Faker::Number.number(0.05)}
  end
end

I receive the error: undefined method 'commissionrate=' for #<Commissionrate:0xbc5ac10>

Any ideas where I'm wrong?