Why the GroundPrimitive's fragment shader didn't work?

[venus]

    Cesium.Ion.defaultAccessToken = 'YOUR TOKEN';

    var worldTerrain = Cesium.createWorldTerrain({
        requestWaterMask: true,
        requestVertexNormals: true
    });

    var viewer = new Cesium.Viewer('cesiumContainer', {
        terrainProvider: worldTerrain
    });
    var rectangleInstance = new Cesium.GeometryInstance({
        geometry: new Cesium.RectangleGeometry({
            rectangle: Cesium.Rectangle.fromDegrees(-179, -89, 180, 89),
            vertexFormat: Cesium.VertexFormat.ALL,
        }),
        id: 'rectangle',
    });
    viewer.scene.primitives.add(new Cesium.GroundPrimitive({
        geometryInstances: rectangleInstance,
        // classificationType: ClassificationType.TERRAIN,
        appearance: new Cesium.EllipsoidSurfaceAppearance({
            aboveGround: true,
            fragmentShaderSource: `
                void main() {
                    gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);  // <= look at here
                }
            `
        }),
    }));

Result:

the color should be yellow, but it isn’t.

When I modify GroundPrimitive to Primitive, it looks good.

How to resolve this problem? Thanks in advanced

[guangc68]

@venus

 var rectangleInstance = new Cesium.GeometryInstance({
        geometry: new Cesium.RectangleGeometry({
            rectangle: Cesium.Rectangle.fromDegrees(-179, -89, 180, 89),
            vertexFormat: Cesium.VertexFormat.ALL,
        }),
        id: 'rectangle',
    });
    viewer.scene.primitives.add(new Cesium.GroundPrimitive({
        geometryInstances: rectangleInstance,
        // classificationType: ClassificationType.TERRAIN,
        appearance: new Cesium.EllipsoidSurfaceAppearance({
            aboveGround: true,
            fragmentShaderSource: `
                void main() {
                    gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);  // <= look at here
                }
            `
        }),
    }));

hello，have you slove this problem