Not sure what this error means - works in Playground. (Error: Enums may not contain stored properties)

import Foundation

enum UIBarButtonStyle {
    case Done
    case Plain
    case Bordered
}

class UIBarButtonItem {

    var title: String?
    let style: UIBarButtonStyle
    var target: AnyObject?
    var action: Selector

    init(title: String?, style: UIBarButtonStyle, target: AnyObject?, action: Selector) {
        self.title = title
        self.style = style
        self.target = target
        self.action = action
    }
}

enum Button {
    case Done(String)
    case Edit(String)

    func toUIBarButtonItem() -> UIBarButtonItem {
        switch self {
        case .Done(let buttonTitle):
            return UIBarButtonItem(title: buttonTitle, style: UIBarButtonStyle.Done, target: nil, action: nil)
        case .Edit(let buttonTitle):
            return UIBarButtonItem(title: buttonTitle, style: UIBarButtonStyle.Plain, target: nil, action: nil)

        }


}

let done = Button.Done("Done")

let doneButton = done.toUIBarButtonItem()