collectionViewのカスタムセルにボタンを一つ配置しようとコードを書いたのですが、ビルドする前にタイトルのようなエラーが出てしまいます。
どなたか解決法をご教示ください
よろしくおねがいします。

ビューコントローラー；

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {

    var collectionView: UICollectionView?

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {
        super.init(nibName: nil, bundle: nil)
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        self.setupSubviews()
        self.autolayoutSubviews()

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

    func setupSubviews() {
        let flowLayout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
        flowLayout.minimumInteritemSpacing = 10.0
        flowLayout.minimumLineSpacing = 10.0
        flowLayout.sectionInset = UIEdgeInsets(top: 10.0, left: 10.0, bottom: 10.0, right: 10.0)
        flowLayout.itemSize = CGSizeMake(300.0, 100.0)

        self.collectionView = UICollectionView(frame: CGRectZero, collectionViewLayout: flowLayout)
        self.collectionView!.translatesAutoresizingMaskIntoConstraints = false
        self.collectionView!.dataSource = self
        self.collectionView!.delegate = self
        self.collectionView!.registerClass(ButtonCell, forCellWithReuseIdentifier: "ButtonCell")　//ここでエラー
        self.collectionView!.backgroundColor = UIColor.whiteColor()
        self.view.addSubview(self.collectionView!)
    }

    func autolayoutSubviews() {
        self.collectionView!.topAnchor.constraintEqualToAnchor(self.view.topAnchor).active = true
        self.collectionView!.leadingAnchor.constraintEqualToAnchor(self.view.leadingAnchor).active = true
        self.collectionView!.trailingAnchor.constraintEqualToAnchor(self.view.trailingAnchor).active = true
        self.collectionView!.bottomAnchor.constraintEqualToAnchor(self.view.bottomAnchor).active = true
    }

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 2
    }


    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("ButtonCell", forIndexPath: indexPath) as! ButtonCell
        cell.backgroundColor = UIColor.yellowColor()
        return cell

    }
}

サブクラス；

import UIKit

class ButtonCell: UICollectionViewCell {

    var button = UIButton(frame: CGRectMake(0, 0, 250, 50));

    override func awakeFromNib() {
        super.awakeFromNib()
    }

    override init(frame:CGRect) {
        super.init(frame: frame)
        self.backgroundColor = UIColor.whiteColor()
        self.setupSubviews()
        self.autolayoutSubviews()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.backgroundColor = UIColor.whiteColor()
        self.setupSubviews()
        self.autolayoutSubviews()
    }



    func setupSubviews() {

        button.backgroundColor = UIColor.grayColor();
        button.layer.masksToBounds = true;
        button.layer.cornerRadius = 20.0;
        button.addTarget(self, action: Selector("btn_click:"), forControlEvents:.TouchUpInside);
        button.setTitle("Tap Me", forState: UIControlState.Normal);
        self.addSubview(button);

    }

    func autolayoutSubviews() {

    }

    override func prepareForReuse() {
        super.prepareForReuse()

    }
}