Sortables: beforeStop

Hi,
I can't seem to get the beforeStop callback function working. If I do
something like:
var options = {
start:function() { alert("start"); }
}
Then I get the alert at the appropriate time. However, if I do:
var options = {
beforeStop:function() { alert("before stop"); }
}
I never see the alert no matter what I do. Does this callback need to
be used in conjunction with other specific properties?