Uncaught TypeError: Cannot set property 'innerHTML' of null at currentTime (JS)

Question

wtf is wrong with following code (output is: Uncaught TypeError: Cannot set property 'innerHTML' of null at currentTime)

function currentTime() {
    var date = new Date();
    var hour = date.getHours();
    var min = date.getMinutes();
    var sec = date.getSeconds();
    var miSec = date.getMilliseconds();

    hour = updateTime(hour);
    min = updateTime(min);
    sec = updateTime(sec);
    miSec = updateTime(miSec);

    document.getElementById("clock").innerHTML = hour + " : " + min + " : " + sec + " : " + miSec;

    var t = setTimeout(function() { currentTime() }, 1000);
}

function updateTime(k) {
    if (k < 10) {
        return "0" + k;
    }
    else {
        return k;
    }
}

currentTime();

```