How to reproduce: Script Error, “Object does not support this property or method.”

var myEl = document.getElementById('myElement');
myEl.appendChild(...)

function myFunc() {
    alert('a');
    var myEl = document.getElementById('myElement');
    myEl.appendChild(something);
    alert('b');
    // more code
}

var blah = {};
blah.methodWhichDoesntExist(); // <-- error