[Moo] About $uid for IE, why array?

[joseanpg]

Hello, I have a question about the storage module in MooTools 1.2.1.
We can see

var $uid = (Browser.Engine.trident) ? function(item){
        return (item.uid || (item.uid = [Native.UID++]))[0];
} : function(item){

        return item.uid || (item.uid = Native.UID++);

};

Why is the array necessary for Trident branch?