greasemonkey script not works vs enhancer for youtube works

Question

I'm a bit disturbed, when i tried this code in youtube website

var str = "My String",
el = document.querySelector("h1");
el.innerHTML = str;

It's work with "custom script" in the firefox's module "Enhancer for Youtube" but doesn't work when i put it in a greasemonkey's script.

Any idea where i'm wrong?

I've tried to add a windows.onload or document.onload before but nothing had worked. I've add an alert before that work on both.