Lost _keys after add OrderedDictionary to an NSMutableArray

[Phong Nguyen]

I don't know how after add an OrderedDictionary instance to NSMutableArray, it lost all key although there're index of individual key without any value. Look at the the following screenshot:
-Before add to NSMutableArray:

-Then, after add to NSMutableArray:

As you can see the same instance OrderedDictionary but after add to array I can't find where are _keys gone?