未捕获的TypeError：无法读取属性＆＃39;值＆＃39; of null（匿名函数）@ function.js：16

Question

我一直在检查Uncaught TypeError的错误：无法读取属性＆＃39; value＆＃39; of null（匿名函数）@ function.js：16

为什么呢？我做错了吗？

  //sets the index varriable to 1
    var indx = 0;
    var sFirstName = [];
    var sLastName = [];
    var sNumber = [];
    var sPoints = [];
    var sFirstName = [];

     //sets text from input to var x from "usernameStudent       
        var v = document.getElementById("indx").value;
        var w = document.getElementById("sFirstName").value;
        var x = document.getElementById("sLastName").value;
        var y = document.getElementById("sNumber").value;
        var z = document.getElementById("sPoints").value;