Sortable: beforeStop

I can't seem to get the beforeStop callback working. If I do this for
example:
var conf = {
stop:function() { alert("it has stopped"); }
}
Then the alert appears when the sorting stops as expected. If however
I do this:
var conf = {
beforeStop:function() { alert("it is about to stop") }
}
I never see the alert. Does this need to be used in conjunction with
other properties?