Myriad pro font issue

Why when i try to type in Arabic with Myriad Pro font in Illustrator Cs6 it show me box with "x" inside it as undefined? but when i type it in illustrator Cs5 i have no problem , i tried everything and can't find a solution i tried to play with Character Style , Glyphs, Preferences > type so every time i want to type in arabic with myriad pro i have to open illustrator Cs5 even if my file on Cs6 Please help