[xwiki-users] class database list property should be empty when creating document

Hi guys, I created a few days ago some documents based on my own class which has databse list properties. In the respective template fields, there was no entry selected when creating the document which is the behaviour I need. When I now create a document based on the same class the first entry is selected a priori and there is no way to deselect, i.e. have nothing selected, it. The old documents still have the ability to deselect the entry in inline editing mode. Can you guys please tell me, what might have happened here or could it be a bug? I dont't understand this apparently undeterministic behaviour. Many thanx in advance. BTW, this is in XWiki 1.7.2. Cheers Sebastian