这两段代码 加载模型能够成功，但是让模型旋转45度则不行，问题出在哪里?

///加载模型
    var modelMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(
        Cesium.Cartesian3.fromDegrees(104.1663, 36.5477, 1680.0));
    var model = scene.primitives.add(Cesium.Model.fromGltf({
        url : '../Apps/SampleData/models/guanlangltf/guanlanfbx2obj2dae.gltf',

        modelMatrix : modelMatrix,
        scale : 0.75
    }));


//初始模型旋转 45度
    model.readyPromise.then(function(argument) {
        var position = Cesium.Cartesian3.fromDegrees(104.1663, 36.5477, 1680.0);
        var mat = Cesium.Transforms.eastNorthUpToFixedFrame(position);
        var rotationX = Cesium.Matrix4.fromRotationTranslation(Cesium.Matrix3.fromRotationZ(Cesium.Math.toRadians(90)));
        Cesium.Matrix4.multiply(mat, rotationX, mat);
        tileset._root.transform = mat;
    }