EXC_BAD_INSTRUCTION on doneBarButton.isEnabled = (newText.length > 0)

Question

My code compiles, but when I try to enter text, the simulator crashes and returns this error. Why?

import Foundation
import UIKit

protocol ListDetailViewControllerDelegate: class {
    func listDetailViewControllerDidCancel(_ controller: ListDetailViewController)
    func listDetailViewController(_ controller: ListDetailViewController, didFinishAdding checklist: Checklist)
    func listDetailViewController(_ controller: ListDetailViewController, didFinishEditing checklist: Checklist)
}

class ListDetailViewController: UITableViewController, UITextFieldDelegate {
    @IBOutlet weak var textField: UITextField!
    @IBOutlet weak var doneBarButton: UIBarButtonItem!

    weak var delegate: ListDetailViewControllerDelegate?

    var checklistToEdit: Checklist?

    override func viewDidLoad() {
        super.viewDidLoad()

        if let checklist = checklistToEdit {
            title = "Edit Checklist"
            textField.text = checklist.name
        }
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        textField.becomeFirstResponder()
    }

    @IBAction func cancel() {
        delegate?.listDetailViewControllerDidCancel(self)
    }

    @IBAction func done() {
        if let checklist = checklistToEdit {
            checklist.name = textField.text!
            delegate?.listDetailViewController(self, didFinishEditing: checklist)
        } else {
            let checklist = Checklist(name: textField.text!)
            delegate?.listDetailViewController(self, didFinishAdding: checklist)
        }
    }

    override func tableView(_ tableView: UITableView, willSelectRowAt indexPath: IndexPath) -> IndexPath? {
        return nil
    }

    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

        let oldText = textField.text! as NSString
        let newText = oldText.replacingCharacters(in: range, with: string) as NSString

        doneBarButton.isEnabled = (newText.length > 0) // EXC_BAD_INSTRUCTION
        return true
    }
}