nil Progress.estimatedTimeRemaining ?

There is no documentation for Progress.estimatedTimeRemaining. I want to get an estimated time to completion from my Progress object, but in my experiments on macOS, it's always nil. What do I need to do to get a non-nil result?

In my case, my progress var is periodically (once per second) updated from the var progress:Float field of a AVAssetExportSession.

Here's what I've got now that's not working: (the progressIndicator is correctly updating)

  @IBOutlet weak var timeRemainingLabel:NSTextField?

  lazy var nsProgress:Progress =  { ()->Progress in
    let prog = Progress(totalUnitCount: 100)

    return prog
  }()

  var progress:Float = 0.0 {
    didSet {
      progressIndicator?.doubleValue = Double(progress)
      nsProgress.completedUnitCount = Int64((progress*100.0).rounded())
      if let time = nsProgress.estimatedTimeRemaining {
        timeRemainingLabel?.stringValue = timeFormatter.string(from: time) ?? ""
      }
    }
  }

  lazy var timeFormatter:DateComponentsFormatter = SongTitledFormSectionsTableCell.newDurationFormatter()

  static func newDurationFormatter()->DateComponentsFormatter {
    let formatter = DateComponentsFormatter()
    formatter.allowedUnits = [.second, .minute]
    return formatter
  }