[jQuery] jQuery form, invalid success label?

Hi,
Firebug is reporting success: is an invalid label with the following code:
$(document).ready(function() {
    // bind 'form2' and provide a simple callback function
    $('#form2').ajaxForm(function() {
        // Server should send data back in json encoding
        dataType:    'json',
        success:    function() {
            alert("DEBUG: Testing.");
        }
    });
});
Any idea?
-sf