Problems with BAPI_USER_GETLIST

Hi there,

i try to use the BAPI_USER_GETLIST

if i use the following expression i get a correct resultlist with my user, firstname 'Dirk'.

ls_selection_exp-parameter = 'USERNAME'.
      ls_selection_exp-option = 'EQ'.
      ls_selection_exp-low = 'C512....'.

if i use the next expression the list is empty.

ls_selection_exp-parameter = 'ADDRESS'.
      ls_selection_exp-field = 'FIRSTNAME'.
      ls_selection_exp-option = 'EQ'.
      ls_selection_exp-low = 'Dirk'.

thx for help

Dirk