[jQuery] :last-child problem

Hi,
I've got strange problem with :last-child selector, my html structure:
<div class="box-full">
<div class="info green">
</div>
<div class="info green">
</div>
</div>
and jquery script:
$(".box-full .info:last-child").addClass("last");
what is strange that
$(".box-full .info:first-child").addClass("last");
works perfect!
thanks in advance
Tomek