HELP ME!!! XAJAX DOES NOT WORK

[eluser]Paganotti[/eluser]
this is the debug of xajax, When I use one of my model'function's I receive this error. Help me please.

Xajax debug output

Mon Jul 16 2007 14:54:59 GMT+0200: Received: <div style="border:1px solid #990000;padding-left:20px;margin:0 0 10px 0;"> <h4>A PHP Error was encountered</h4> <p>Severity: Notice</p> <p>Message: Undefined property: Ajaxtest::$officina_app_model</p> <p>Filename: controllers/ajaxtest.php</p> <p>Line Number: 32</p> </div>
Mon Jul 16 2007 14:54:59 GMT+0200: Calling test_function uri=http://localhost/CodeIgniter/index.php/ajaxtest (post:xajax=test_function&xajaxr=1184590499631&xajaxargs[]=3)
Mon Jul 16 2007 14:54:59 GMT+0200: Initializing Request Object..
Mon Jul 16 2007 14:54:59 GMT+0200: Starting xajax...