Why link error with following simple OpenGL ES2 shader?

Just simple programming with follow(from here)

float odd = floor(mod(gl_FragCoord.y, 2.0));  // prob开发者_JAVA技巧lems come from this line 
gl_FragColor = vec4(v_color.x, v_color.y, v_color.z, odd);

But it can not pass the link.

I know it is quite simple,but it is quiet new for me, I just want to go through this problem. Thanks for letting me know.