DnD. Allow dropping only on droppable elements

I wrote such code:
***
$(document).ready(function() {
$(".draggable").draggable({
revertDuration:1000,
scope:'board',
opacity: 0.50,
snap:true,
snapMode:'inner'
});
$(".blackPiece").droppable({
accept: ".draggable",
scope:'board'
});
});
****
I thought that this snippet would prevent dropping dragged elements on
"not-droppable" ones. But it is not. How can I make my plans in life?
=)