Ich bin ein wenig verwirrt, warum dieser Code nicht funktioniert!

Das HTML-Markup:

<div id="diva"><b>Score</b> some <i>goals</i></div>
<div id="soda"></div>

Der JavaScript-Code:

function GetSelectedText () {
if (window.getSelection) {  // all browsers, except IE before version 9
    var range = window.getSelection ();
    alert (range.toString ());
} 
else {
    if (document.selection.createRange) { // Internet Explorer
        var range = document.selection.createRange ();
        alert (range.text);
    }
}
}

var butn = document.getElementById("soda");
butn.onclick = function(){
    GetSelectedText();
}