How to fix this error in HTML (no error shown)

My function createBomb() is not working because none of the elements are turning into bombs.

I've tried finding the error.

HTML:

<div class = "grid-square" onclick = "check(1,8)" id = "18">?</div>

Javascript:

function createBomb() {
    bombNum = Math.floor(Math.random() * 80) + 1;
    if (!document.getElementById(bombNum).classList.contains("bomb")) {
        document.getElementById(bombNum).classList.add("bomb");
    } else {
        createBomb();
    }
}
createBomb();

I expect the function createBomb() to make a random div a bomb.