Nesting drag and drop element problem

Nesting drag and drop element problem

I am having a problem nesting a drag and drop element.  I have a jsfiddle link below.

I am trying to build a form via drag and drop.  I drag a "Fieldset" from the left menu to the black working area.  I can drag multiple then sort them to reorder.  I want to then drop fields in to the fieldsets.  The field is located in the left menu below the fieldsets.  I want to drag the field from the menu to the working area and inside the dropped/sortable fieldsets already in the working space.

When I drag the field to the fieldset, it does not append to the fieldset in the working area.  Can someone help me understand what I am doing wrong?