[TYPO3-dev] Non-selectable Access field divider

[ktt]

Hello,

I try to add my custom dividers to fe_group Acess field
in BE extension, like this one "__Usergroups:__".
After print_r($params['items']), I can see items are 

in this format:         [3] => Array

(
[0] => Lemmons
[1] => 7
[2] => empty-empty
)
But there are no dividers among them. I have tried to
add a custom divider: 
$dividerArray = array($key,'','');
But it becomes selectable, while default
"__Usergroups:__" is not.
Must dividers have their own special values to become unselectable?

Thank you in advance,
Kast A.