Problem applying constraint to UIStackView

TestVC.swift

//  File.swift
//  Medicaments
//
//  Created by Joan Pujol Espinar on 22/11/2016.
//  Copyright © 2016 Joan Pujol. All rights reserved.
//

import UIKit

class TestVC : UIViewController {
    var scrollView: UIScrollView!
    var stackView: UIStackView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        scrollView = UIScrollView()
        scrollView.backgroundColor = UIColor.cyan
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(scrollView)
        view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|[scrollView]|", options: .alignAllCenterX, metrics: nil, views: ["scrollView": scrollView]))
        view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|[scrollView]|", options: .alignAllCenterX, metrics: nil, views: ["scrollView": scrollView]))
        
        
        stackView = UIStackView()
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.axis = .vertical
        stackView.spacing = 15
        //stackView.backgroundColor = UIColor.gray
        scrollView.addSubview(stackView)
        
        scrollView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|[stackView]|", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: ["stackView": stackView]))
        scrollView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|[stackView]|", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: ["stackView": stackView]))

        for _ in 1 ... 20 {
            let textLabel = UILabel()
            textLabel.backgroundColor = UIColor.yellow
            textLabel.text  = "Hi World xxxx"
            stackView.addArrangedSubview(textLabel)
            textLabel.widthAnchor.constraint(equalTo: stackView.widthAnchor).isActive = true //Doesn't work as expected
            //textLabel.widthAnchor.constraint(equalTo: scrollView.widthAnchor).isActive = true //Work as expected
        }
    }
    
}