Android OpenGL ES 2.0：无效的顶点着色器。链接无法继续

uniform mat4 u_mvpMatrix;
uniform mat4 u_mvMatrix;
uniform vec3 u_lightPosition;

attribute vec4 a_position;
attribute vec4 a_color;
attribute vec3 a_normal;

varying vec4 v_color;

void main() {
    vec3 modelViewVertex = vec3(u_mvMatrix * a_position);
    vec3 modelViewNormal = vec3(u_mvMatrix * vec4(a_normal, 0.0));

    float distance = length(u_lightPosition – modelViewVertex);
    vec3 lightVector = normalize(u_lightPosition – modelViewVertex);
    float diffuse = max(dot(modelViewNormal, lightVector), 0.1);
    diffuse = diffuse * (1.0 / (0.25 * distance * distance));

    v_color = a_color * diffuse;

    gl_Position = mvpMatrix * a_position;
}