cesium加载.gltf数据出现的问题

我想请问下，加载gltf数据报这个错，这个是什么问题？ message: "Failed to load model: ./CesiumMilkTruck.gltf↵Unexpected token < in JSON at position 0" name: "RuntimeError" stack: "Error↵ at new RuntimeError (http://localhost:8080/js/about.js:72566:19)↵ at http://localhost:80

代码： var viewer = new Cesium.Viewer('cesiumContainer', {

    geocoder: false,
    homeButton: false,
    sceneModePicker: false,
    baseLayerPicker: false,
    fullscreenButton: false,
    scene3DOnly: true, //如果设置为true，则所有几何图形以3D模式绘制以节约GPU资源
    navigationInstructionsInitiallyVisible: false,
    navigationHelpButton: false,
    animation: false,
    timeline: false,
    fulllscreenButtond: false,
    vrButton: false,
    infoBox: false
});

//加载gltf格式数据到cesium
var scene = viewer.scene;

var position = Cesium.Cartesian3.fromDegrees(123.2856, 41.76692, 0);
var heading = Cesium.Math.toRadians(135);
var pitch = 0;
var roll = 0;
var hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);
var orientation = Cesium.Transforms.headingPitchRollQuaternion(position, hpr);

var entity = viewer.entities.add({
    position: position,
    orientation: orientation,
    model: {
        uri: "./CesiumMilkTruck.gltf",
        minimumPixelSize: 128,
        maximumScale: 20000
    }
});
viewer.trackedEntity = entity;