replace() call failed with CKEditor

Hello,
I have a JS issue with dhtml_menu.js on node edit form when I want to edit Arabic content. It prevents the load of the CKEditor on the page.
Uncaught TypeError: Cannot read property 'replace' of undefined - dhtml_menu.js?:65
The line error is:
var rtl = $('html').attr('dir') == 'rtl' ? Math.ceil($('.menu li').css('margin-right').replace('px', '')) + 1 : 0;
So it happens only on the arabic UI because on other languages I used, this condition $('html').attr('dir') == 'rtl' is false, the 0 is used and the replace() function is never called.
I can quickly create a patch to bypass this code or put a reliable value on the rtl var, but I was wondering if I could face real troubles if this is not well managed? As we verified if the dir of the page is rtl, it should have a good reason I guess?