rails 4.1 JSON API problem

gistfile1.rb

class ApplicationController < ActionController::Metal
  include ActionController::Helpers
  include ActionController::Redirecting
  # Rendering MUST be before Renderers::All
  include AbstractController::Rendering
  include ActionController::Renderers::All
  # this is now in ActionView::Layouts:
  # include AbstractController::Layouts
  include ActionView::Layouts

  include ActionController::ConditionalGet
  include ActionController::MimeResponds
  include ActionController::RequestForgeryProtection # not sure I need this
  include ActionController::ForceSSL
  include AbstractController::Callbacks
  include ActionController::Instrumentation
  include ActionController::ParamsWrapper
  include ActionController::StrongParameters
  include ActionController::Serialization # for active_model_serializers
  include Rails.application.routes.url_helpers
  
  # some stuff
end

class VouchersController
  respond_to :json
  
  def update
    # do some stuff
    render json: response.to_json, status: 422
  end
end