weakreference bug?

[testmethod]
public vacant memory()
{
    var wr = new weakreference("aaabbb");
    assert.istrue(wr.isalive);
    gc.collect();
    gc.collect();
    gc.collect();
    gc.collect();
    gc.collect();
    assert.isfalse(wr.isalive); //<-- fails here
}

it's .net 3.5 sp1

can anyone tell me since exam fails?

edit: interjection stusmith

you have anxiety string,
given constant, is
substantially interned (ie dynamically
allocated), never be
collected.

that it. altered initial line

var wr = new weakreference(new object());

and exam passes :-)