Issues encountered while passing uniform values

CesiumJS
1

I wish to pass a float type value of ‘u_time’ from ‘material’ into ‘fragment shader’ as a uniform. However, when ‘u_time’ is utilized in ‘fragment shader’, an error occurs. I have provided the primary code and error message below. Could you identify any mistakes in this transfer method?

let material = new Cesium.Material({
    fabric: {
        type: 'Flow',
        uniforms: {
            image: '../image/blank.jpg',
            time: 0.0,
            u_time: 0.0,
        },
        source: `
        czm_material czm_getMaterial(czm_materialInput materialInput) {
                czm_material material = czm_getDefaultMaterial(materialInput);
                ...
                material.diffuse = texture2D(image, newCoords).rgb;
                return material;
            }
                `
    }
});

let fs = getFS();

function getFS() {
    source = `
    uniform float u_time;
    void main() {
        czm_materialInput materialInput;
        vec2 uv = gl_FragCoord.xy;
        uv.y += u_time;
    }
    `;
    return source;
}

viewer.scene.primitives.add(new Cesium.Primitive({
    geometryInstances: coneInstance,
    appearance: new Cesium.Appearance({
        material: material,
        renderState: {
            blending: Cesium.BlendingState.ALPHA_BLEND,
            depthTest: {
                enabled: true
            },
            cull: {
                enabled: true,
                face: Cesium.CullFace.BACK
            },
            depthMask: true,
        },
        fragmentShaderSource: fs,
        vertexShaderSource : vs,
    }),
}));

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