Custom table view cell: IBOutlet label is nil

Consider the following simple view controller:

class ViewController: UIViewController, UITableViewDataSource {
    @IBOutlet weak var tableView: UITableView!

    var items = ["One", "Two", "Three"]

    override func viewDidLoad() {
        super.viewDidLoad()

        self.tableView.registerClass(CustomTableViewCell.self, forCellReuseIdentifier: "customCell")
        self.tableView.dataSource = self
    }

    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return self.items.count;
    }

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = self.tableView.dequeueReusableCellWithIdentifier("customCell") as CustomTableViewCell
        cell.titleLabel!.text = self.items[indexPath.row]
        return cell
    }
}

And custom cell view:

class CustomTableViewCell: UITableViewCell {
    @IBOutlet weak var titleLabel: UILabel!   
}

This code causes the following error.

fatal error: unexpectedly found nil while unwrapping an Optional value

titleLabel is nil — it's not clear why. Setting default properties of UITableViewCell (like textLabel) work just fine.

I'm using a nib for the custom cell.

Both the labels and table views are correctly connected to their IBOutlets.

Both the prototype cell and the custom nib view are marked as having a CustomTableViewCell class.

I'm new to iOS development, am I missing something obvious?

Sample Xcode project available.