address structure's component dynamically

Dear All,

I am getting the message "The data object ls_udat3 does not have a component called <fs_genfield>"
when I am trying to address structure's component dynamically. I kindly ask you for your support,
What have I done wrong at the point in the program. 

Best regards

Alex

data:  ls_udata type zbbpusr01_2,
         lt_attr_data  type bbpt_attr,
         ls_attr_data  type bbps_attr,
         ls_vlist      type bbps_attr_values,
         lt_vlist      type bbpt_attr_values,
         ls_udat3      type zbbpusr01_3,
         ls_udat3_2    type zbbpusr01_3,
         lv_fieldname  type string,
         lt_udat3      type table of zbbpusr01_3,
         et_fcat       type lvc_t_fcat,
         lv_user type baluser.

  LOOP AT p_gt_udate INTO ls_udata.
    lv_user = ls_udata-userid.

    CALL FUNCTION 'BBP_READ_ATTRIBUTES'
      EXPORTING
        iv_user                 = lv_user
        IV_SCENARIO             = 'BBP'
        it_attr_list            = gt_attr_list2
      IMPORTING
        ET_ATTR                 = lt_attr_data
      EXCEPTIONS
        object_id_not_found     = 1
        no_attributes_requested = 2
        attributes_read_error   = 3
        OTHERS                  = 4.

    FIELD-SYMBOLS:
    <fs_zbbpusr03>  type zbbpusr01_3,
    <fs_zbbpusr>    type zbbpusr01_3,
    <fs_field>      type bbps_attr-attr_id,
    <fs_attr_data>  type bbps_attr,
    <fs_vlist>      type bbps_attr_values,
    <fs_genfield>   type any.

    loop at lt_attr_data ASSIGNING <fs_attr_data>.
      assign <fs_attr_data>-attr_id to <fs_genfield>.
      loop at <fs_attr_data>-vlist  ASSIGNING <fs_vlist>.
        MOVE <fs_vlist>-value to ls_udat3-<fs_genfield>.
      ENDLOOP.
    ENDLOOP.
  ENDLOOP.
ENDFORM.                    " READ_ATTR