Capybara with js: true not finding checkbox

I need to test if Javascript alert is being displayed in validation proccess after checking one checkbox. I'm using RSpec, Capybara with Webkit and Database Cleaner.

My test without JS: true :

  it "alerts to choose two players" do
    set_and_visit
    first("input[type='checkbox']").set(true)
    expect(page).to have_content('Remember that you must choose two players to start a game.')
  end

Finds a checkbox but cannot find the alert message.

When I add JS - "alerts to choose two players", js: true do it returns an error

NoMethodError:
       undefined method `set' for nil:NilClass

I've tried using check 'John Doe' and first("input[type='checkbox']").check but it still didn't work.