jQuery 1.4.2 / jQuery UI droppable - remove(); issue with Internet Explorer or something wrong?

Question

Hi, I'm using jQuery 1.4.2 (j is the .noConflict() ) / jQuery UI 1.8.5 and I'm experiencing a problem with the following code.

This runs well in Chrome, FireFox and Safari, but none in Internet Explorer. The alert(); fires but the following line (the remove(); ) no.

XHTML markup:

<div class="mainarea">
    <div class="dnd">
        <div class="person dad"></div>
        <div class="person mum"></div>
    </div>
</div>

<div class="tools">
   <div class="person dad"></div>
   <div class="person mum"></div>
   <div class="person boy"></div>
   <div class="person girl"></div>
   <div class="bin"></div>
</div>

Javascript code:

j(document).ready(function(){

    // make the source item draggable
    j('.tools .person').draggable({
        revert: "invalid", 
        helper: "clone"
    });

    // the target drag n'drop area
    j('.dnd').droppable({
            accept: ".tools > .person",
            revert: "invalid", 
            activeClass: "active",
            drop: function( event, ui ) {
                //copy from source and make it replaceable by another one
                var obj = ui.draggable.clone().droppable({  hoverClass: "active",   accept: ".tools .person"    });

                // in case of replace
                if( j(".dnd > .person.active").size() )
                    j(".dnd > .person.active").replaceWith( obj );
                else   // in case of new or limit reached
                    if( (j(".dnd > .person.active").size() == 0) && (j(".dnd > .person").size() < 4) )
                        obj.appendTo('.dnd');
            }
        });

    // the bin to delete the selected persons
    j('.bin').droppable({
            accept: ".dnd > .person",
            hoverClass: "active",
            drop: function( event, ui ) {
                            alert('debug');
                ui.draggable.remove();
            }
        });

    // makes drag n'drop is sortable
    j(".dnd").sortable({    placeholder: 'empty'    });

    //helpers
    j(".dnd").disableSelection();

});

Can somebody help me? Thanks.