[jQuery] find is finding what?

[Trans]

Hi, I have the following:

    var stack = $('#mydiv')
    var last = stack.find('.card:last-child');

It doesn't make sense to me, b/c even if stack has no children, last
is being assigned to some object irregardless of the '.card' filter.
To work around I had to do this next:

    if (last.is('.card')) {
      return last;
    } else {
      return null;
    };

What am I misunderstanding?

Thanks,
T.