getItemIndex не работает!

[Aleksey123]

var myArray :ArrayCollection = new ArrayCollection
([{name:"field1",dat:"AZ"},{name:"field2",dat:"MA"},{name:"field3",dat:"MZ"}]);

var dt:Object = {name:"field1", dat:"AZ"};

trace(myArray.getItemIndex({name:"field2", dat:"MA"}));// -1
trace(myArray.getItemIndex(dt));// -1

trace(myArray.getItemAt(0) == {name:"field1",dat:"AZ"});//false
trace(myArray.getItemAt(0) == dt);//false

Почему выводит -1?