click event doesn’t change font size [duplicate]

I have this code:

<!DOCTYPE html><html><head></head><body>

<button id="b12">12px</button>
<button id="b14">14px</button>
<button id="b16">16px</button>

<script>
document.getElementById('b12').onclick = clickHandler(12)
document.getElementById('b14').onclick = clickHandler(14)
document.getElementById('b16').onclick = clickHandler(16)
function clickHandler(size) {
    return function() {
        document.body.style.fontSize = `${size}px`
    }
}
</script>

</body></html>

But the buttons font size doesn’t seem to change when I click one.
I also made a codepen here:
https://codepen.io/trufo/pen/eYedaLE
If anyone could tell me what I’m missing in that code, that would be greatly appreciated.