draggable error - Object doesn't support this property or method

draggable error - Object doesn't support this property or method


Hello,
I am trying to get drag/drop functionality working in my app and I am
getting the "Object doesn't support this property or method" error on
the following line of code:
$(".dragDropSource").draggable({helper: 'clone'});
Here is the full function along with the js files that I am including:
<script type="text/javascript" src="/js/ui.core.js"></script>
<script type="text/javascript" src="/js/ui.draggable.js"></script>
<script type="text/javascript" src="/js/ui.droppable.js"></script>
<script type="text/javascript">
$(document).ready(function(){
//Register Sources
$(".dragDropSource").draggable({helper: 'clone'});
//Register Targets
$(".dragDropTarget").droppable({
     accept: ".dragDropSource",
     activeClass: 'droppable-active',
     hoverClass: 'droppable-hover',
     drop: function(ev, ui) {
         $(this).append("
Dropped!");
     }
});
});
Does anyone have any idea what I'm missing? Thanks in advance.