CustomMaterials don't work when used in an importScene

Window {
    visible: true
    height: 800
    width: 600    
    Node {
        id: scene
        PerspectiveCamera {
            z: 300
        }        
        DirectionalLight {
        }        
        Model {
            source: "#Cube"
            material: CustomMaterial { }
        }
     }    
     View3D {
        anchors.fill: parent
        importScene: scene
     }
}