jquery ui menu links are not working when user press enter key

jquery ui menu links are not working when user press enter key

Hi,
I create a menu and try to make it working by using keyboard.
I can use tab key to navigate through the menu from top level to submenu, but when I press enter key, the submenu disapeared and the link is not working (when I use mouse to click the link, it worked well).
 
And I also create a UL list, and the links are working by pressing enter keycode. 
Thanks,
Ying