Minimum height for UICollectionView (Swift)

Question

Im trying to create a simple UICollectionView in Swift with a custom UICollectionViewCell. I have the following method in my UICollectionViewController to set the size of a cell;

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let width  = (collectionView.frame.width  / 3) - 10
    let height = CGFloat(170)

    return CGSize(width: width, height: height)
}

I then set constraints for the cell as follows;

private func setImageViewConstraints() {
    itemImageView.translatesAutoresizingMaskIntoConstraints = false // enables Auto-Layout

    itemImageView.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true      // center horizontally
    itemImageView.topAnchor.constraint(equalTo: self.topAnchor, constant: 8).isActive = true // 8pt from top of cell

    // 1:1 Aspect Ratio
    itemImageView.widthAnchor.constraint(equalToConstant: (self.frame.width - 4)).isActive = true
    itemImageView.heightAnchor.constraint(equalTo: itemImageView.widthAnchor).isActive = true
}

private func setupLabelConstraints() {
    itemLabel.translatesAutoresizingMaskIntoConstraints = false

    itemLabel.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true

    itemLabel.leftAnchor.constraint(equalTo: itemImageView.leftAnchor, constant: 0).isActive = true
    itemLabel.rightAnchor.constraint(equalTo: itemImageView.rightAnchor, constant: 0).isActive = true

    itemLabel.topAnchor.constraint(equalTo: itemImageView.bottomAnchor, constant: 4).isActive = true
    itemLabel.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: -4).isActive = true
}

However, the height of the cell varies, depending on the height of it's content, for example;

How can I set the height of the cells so they are all the same? Thanks!