Correct way to require Payment API in Rails

I'm trying to integrate Mollie Payments using the Mollie API I added the gem to my gemfile and ran bundle install. After that i did this to my existing controller:

class PagesController < ApplicationController

  require 'Mollie/API/Client'

  def form_page
    mollie = Mollie::API::Client.new
    mollie.api_key = 'test_SKQzVUv7Rs7AUqmW7FdTvF9SbEujmQ'

    payment = mollie.payments.create(
        amount: 10.00,
        description: 'My first API payment',
        redirectUrl: '/index'
    )

    payment = mollie.payments.get(payment.id)

    if payment.paid?
      puts 'Payment received.'
    end
  end

  def success
  end
end

Where form_page is the 'new' method and 'success' is the page where it should redirect to after payment has succeeded.

But when going to the form_page view i get this error:

NoMethodError (undefined method `api_key=' for #<Mollie::API::Client:0x007fa6fb8>)

So my guess is that the API isn't required the right way or something like that. Anybody has an idea on what i'm doing wrong? Any help would be much much appreciated!!