NSCollectionView viewForSupplementaryElementOfKind не вызывается

В моем NSCollectionViewItem я установил код ниже.

func collectionView(_ collectionView: NSCollectionView, viewForSupplementaryElementOfKind kind: NSCollectionView.SupplementaryElementKind, at indexPath: IndexPath) -> NSView {
    let view = collectionView.makeSupplementaryView(ofKind: .sectionHeader, withIdentifier: NSUserInterfaceItemIdentifier(rawValue: "sectionHeader"), for: indexPath)
    view.wantsLayer = true
    view.layer?.backgroundColor = NSColor.red.cgColor
    return view       
}

func collectionView(_ collectionView: NSCollectionView, layout collectionViewLayout: NSCollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> NSSize {
    return NSSize(width: self.frame.size.width, height: 60)
}

Однако, когда я помещаю print() в функцию viewForSupplementaryElementOfKind, он никогда не выполняется. Кроме того, не отображается красный заголовок.

Вот как я настроил NSCollectionView

public let collectionView: NSCollectionView = {
    let v = NSCollectionView(frame: NSRect.zero)
    v.wantsLayer = true
    v.layer!.backgroundColor = NSColor.clear.cgColor
    v.isSelectable = true
    v.backgroundColors = [NSColor.clear]
    v.allowsEmptySelection = false
    v.collectionViewLayout = {
        let l = NSCollectionViewFlowLayout()
        l.minimumInteritemSpacing = 1
        l.minimumLineSpacing = 1
        l.scrollDirection = NSCollectionView.ScrollDirection.vertical
        l.sectionInset = NSEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        return l
    }()

    return v
}()

Что я делаю не так?