getSelectionRange() in CKEditor

I have found a very useful function for textarea

function getSelectionRange(oElm)
{
   var r = { text: '', start: 0, end: 0, length: 0 };
   // W3C/Gecko
   if (oElm.setSelectionRange)
   {
      r.start= oElm.selectionStart;
      r.end = oElm.selectionEnd;
      r.text = (r.start != r.end) ? oElm.value.substring(r.start, r.end): '';
   }
   // IE
   else if (document.selection)
   {
      if (oElm.tagName && oElm.tagName === 'TEXTAREA')
      {
         var oS = document.selection.createRange().duplicate();
         var oR = oElm.createTextRange();
         var sB = oS.getBookmark();
         oR.moveToBookmark(sB);
      }
      else
         var oR = document.selection.createRange().duplicate();

      r.text = oR.text;
      for (; oR.moveStart('character', -1) !== 0; r.start++);
      r.end = r.text.length + r.start;
   }
   r.length = r.text.length;
   return r;
}

Its working fine with a standard textarea

$('#button').click(function()
{
   var obj = document.getElementById('editor1');
   console.log(getSelectionRange(obj));
});

now I use editor = CKEDITOR.replace('editor1',.....

$('#button').click(function()
{
   var obj = editor;
   //var obj = CKEDITOR.instances['editor1'];
   console.log(getSelectionRange(obj));
});

But now I get only Object { start=0, end=0, length=0, text=""}

What is my mistake? Can anybody help me?