The ios app I am currently developing starts off on a login screen which then presents the next view controller via a login button. Upon moving on to the next VC I get the error: Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors <NSLayoutYAxisAnchor:0x600000d0af40 "UIStackView:0x7fe87df14160.top"> and <NSLayoutYAxisAnchor:0x600000d0ad40 "UILayoutGuide:0x600002102300'UIViewSafeAreaLayoutGuide'.bottom"> because they have no common ancestor. Does the constraint or its anchors reference items in different view hierarchies? That's illegal.' Here is the relevant VC code for the contoller being showed: class LocationSelectViewController: UIViewController {       let promptLabel: UILabel = {     let label = UILabel()           label.translatesAutoresizingMaskIntoConstraints = false     label.text = "Enter Your Location`s \nAddress"     label.font = .boldSystemFont(ofSize: 23)     label.numberOfLines = 0     label.preferredMaxLayoutWidth = label.frame.width     label.textAlignment = .center           return label   }()       let streetAddressField: UITextField = {     let textField = UITextField()         textField.translatesAutoresizingMaskIntoConstraints = false     textField.placeholder = "street address"     textField.borderStyle = .roundedRect     textField.textAlignment = .left         return textField   }()       let cityField: UITextField = {     let textField = UITextField()         textField.translatesAutoresizingMaskIntoConstraints = false     textField.placeholder = "city"     textField.borderStyle = .roundedRect     textField.textAlignment = .left         return textField   }()       let stateField: UITextField = {     let textField = UITextField()         textField.translatesAutoresizingMaskIntoConstraints = false     textField.placeholder = "state"     textField.borderStyle = .roundedRect     textField.textAlignment = .left         return textField   }()       let saveButton: UIButton = {     let button = UIButton(type: .system)     button.setTitle("Add Location", for: .normal)     button.translatesAutoresizingMaskIntoConstraints = false           return button   }()       var currentUser: User?   let geocoder = CLGeocoder()   override func viewDidLoad() {     super.viewDidLoad()           view.backgroundColor = .white           let stackView = UIStackView()           let views: [UIView] = [streetAddressField, cityField, stateField]           stackView.axis = .vertical     stackView.spacing = 17     stackView.translatesAutoresizingMaskIntoConstraints = false           view.addSubview(promptLabel)     view.addSubview(stackView)     view.addSubview(saveButton)           for view in views {       stackView.addArrangedSubview(view)     }           let constraintsArray: [NSLayoutConstraint] = [promptLabel.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor), promptLabel.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 30), promptLabel.heightAnchor.constraint(lessThanOrEqualToConstant: 70), stackView.topAnchor.constraint(equalTo: promptLabel.bottomAnchor, constant: 40), stackView.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor), stackView.widthAnchor.constraint(equalToConstant: 300), saveButton.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor), saveButton.topAnchor.constraint(equalTo: stackView.bottomAnchor, constant: 30)]           saveButton.addTarget(self, action: #selector(addLocationTouchUpInside), for: .touchUpInside)           NSLayoutConstraint.activate(constraintsArray)   } Also I am aware that the constraints are hard to read so they are listed below promptLabel.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor), promptLabel.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 30), promptLabel.heightAnchor.constraint(lessThanOrEqualToConstant: 70), stackView.topAnchor.constraint(equalTo: promptLabel.bottomAnchor, constant: 40), stackView.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor), stackView.widthAnchor.constraint(equalToConstant: 300), saveButton.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor), saveButton.topAnchor.constraint(equalTo: stackView.bottomAnchor, constant: 30) I orginally asked this question on SO which has lead me to believe this is a potential bug. I likely could hold off or workaround the issue by rewriting/choosing different constraints, but if its a bug I would much rather not put in pointless work.