calling a php script using $.POST in javascript

I have HTML form with a textarea and a button which calls the function below when a part of the text is selected. With de id of the clicked button and the selected text I want to call a php script by the POST method.

$(document).ready(function() {
    $(".postbit_buttons").on("click", ".quoted_clicked", function() {
        var post = this.id;
        var selected = GetSelectedText();
        alert(" post = " + post + " seleted text = " + selected);
        $.post("test_quoted.php", {
                pid: post,
                selection: selected
            })
            .done(function() {
                alert("done")
            });
    });
});

Function GetSelectedText() to get the selection was found here. The first alert is shown with the correct information. However, after clicking OK I get the following error message on the browser console:

TypeError: 'collapse' called on an object that does not implement interface Selection.

I have used a similar construct in another part of the forum software of which this is a part, and that works.

I have pain in my eyes of staring at this, but cannot find the cause. Is there anybody who can help me on this?

@Rory The code of function GetSelectedText() is:

function GetSelectedText()
{
  var selectedText=(
        window.getSelection
        ?
            window.getSelection()
        :
            document.getSelection
            ?
                document.getSelection()
            :
                document.selection.createRange().text
     );
 if(!selectedText || selectedText=="")
 {
    if(document.activeElement.selectionStart)
    {
     selectedText = document.activeElement.value.substring(
          document.activeElement.selectionStart
          . document.activeElement.selectionEnd);
    }
 }
 return selectedText;
}

Thanks to Rory's question I tried things out. I discovered that is was the variable "selected" in which the selected text is stored was the cause of the error. I now use the following version of GetSelectedText():

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

and it works!! Sorry for bothering you. I do practice programming since 1968, but almost 100% was scientific computing with FORTRAN. Javascript is very new to me.

Regards, Ad Bakker