Applying a Behavior on an attached property asserts

import QtQuick 2.1
import QtQuick.Layouts 1.1

Item {
    width: 300
    height: 400
    
    RowLayout {
        anchors.fill: parent
        Rectangle {
            color: "red"
            Layout.preferredWidth: 50
            Layout.fillHeight: true
        }
        Rectangle {
            color: "yellow"
            Layout.fillWidth: true
            Layout.fillHeight: true
        }
        Behavior on Layout.preferredWidth {
            NumberAnimation { duration: 1000 }
        }
    }
}