movieclip isn't private dictionary

i have collection where i reason information movieclips, i wish information rubbish collected i stop controlling movieclips. i'm controlling noxious keys parameters, works ideally data, however i've run problem.

this formula works great:

var mc = new movieclip();
var dic = new dictionary(true);
dic[mc] = 12;
mc = null;
system.gc();
system.gc();
for (var obj dic)
   trace(obj);    //this doesn't execute

but i indeed movieclip, stops working:

var mc = new movieclip();
var dic = new dictionary(true);
dic[mc] = 12;
addchild(mc);
removechild(mc);
mc = null;
system.gc();
system.gc();
for (var obj dic)
   trace(obj);    //this prints [object movieclip]

why does happen? something i'm doing wrong? there workaround?

edit: i know specific instance i delete dic[mc], nonetheless impetus simplified case. general, i don't wish manually have mislay movieclip dictionary, nonetheless should involuntary i don't anxiety anymore rest application.

edit2: i attempted contrast aaron said, came adult only uncanny stuff... only iterating collection (without doing anything) changes behaviour:

var mc = new movieclip();
var dic = new dictionary(true);
dic[mc] = 12;
addchild(mc);
removechild(mc);
mc = null;

for (var objeto dic) {}    // <-- try commenting out line

addeventlistener('enterframe', f);  // i imitation essence each frame, if
                                    // gets private after awhile

function f(evento)
{
    system.gc();
    system.gc();

    (var objeto dic)
        trace(objeto);
}

this keeps duplicate [object movieclip] each frame, unless i critique out indicated line, where doesn't imitation anything.