Right-click selection

I have added an item to the context menu, and I have it firing a function that is supposed to alter the selected text (ie. the text that the cursor was over during the right-click). The code I have below is not collecting the selected test, however. Do I need to do something different? Does the text have to be highlighted, or can it simply be right-clicked on?

if(CKEDITOR.env.ie) {
     selectedText = CKEDITOR.currentInstance.getSelection().document.$.selection.createRange().text;
}
else {
     selectedText = CKEDITOR.currentInstance.getSelection().getNative();
}