app starts fine, tap button and picker shows, start to scroll picker, immediate crash

main_view_controller.rb

class MainViewController < UIViewController

  def viewDidLoad
    show_picker_button = UIButton.buttonWithType UIButtonTypeRoundedRect
    show_picker_button.setTitle "Show Year Picker", forState: UIControlStateNormal
    show_picker_button.frame = [[100, 100], [100, 50]]
    show_picker_button.addTarget(self,
                             action: :show_year_picker,
                             forControlEvents: UIControlEventTouchUpInside)
    self.view.addSubview show_picker_button
  end

  def show_year_picker
    year_picker = YearPickerViewController.new
    self.view.addSubview year_picker.view
  end
end

year_picker_view_controller.rb

class YearPickerViewController < UIViewController
  def viewDidLoad
    picker = UIPickerView.new
    picker.backgroundColor = UIColor.whiteColor
    picker.delegate = self
    picker.dataSource = self
    picker.showsSelectionIndicator = true
    self.view.addSubview picker
  end

  def numberOfComponentsInPickerView pickerView
    1
  end

  def pickerView(pickerView, numberOfRowsInComponent:component)
    50
  end

  def pickerView(pickerView, titleForRow:row, forComponent:component)
    (2000 + row).to_s
  end
end