in Admin mode search_fields error

[fatbobman]

i can see the search_fields in admin mode.but i can't use it.
error message:
There's been an error:

Traceback (most recent call last):

  File "C:\Python24\lib\site-packages\django\core\handlers\base.py",
line 71, in get_response
    response = callback(request, **param_dict)

  File
"C:\Python24\lib\site-packages\django\contrib\admin\views\decorators.py",
line 49, in _checklogin
    return view_func(request, *args, **kwargs)

  File
"C:\Python24\lib\site-packages\django\contrib\admin\views\main.py",
line 154, in change_list
    for field_name in lookup_opts.admin.search_fields:

TypeError: iteration over non-sequence