QML comma operator property lookup failure

import QtQuick 2.7
import QtQuick.Window 2.2

Window {
  id: root

  property var testObject: ({ 'prop': true })

  width: 400
  height: 200

  Item {
    id: innerItem

    property int counter
    //property bool active: innerItem.counter, testObject.prop	            // works
    //property bool active: counter !== undefined ? testObject.prop : false // works
    property bool active: counter, testObject.prop

    anchors.fill: parent

    Timer {
      id: timer
      interval: 1000
      running: true
      repeat: true
      onTriggered: {
        testObject.prop = !testObject.prop
        ++innerItem.counter
      }
    }

    Text {
      anchors.centerIn: parent
      text: innerItem.active
    }
  }
}