error in firefox with markitup

I get the following error when I use MarkItUp Editor,

 uncaught exception: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMNSHTMLTextAreaElement.select开发者_运维问答ionStart]"  nsresult: "0x80004005 (NS_ERROR_FAILURE)"  location: "JS frame :: /jquery.markitup.js :: get :: line 418"  data: no]

the line is

 caretPosition = textarea.selectionStart;

I searched the internet but found nothing that helps me. The problem is in firefox 3.6 and 4.1.