Assign type Conflict

FIELD-SYMBOLS: <l_table> TYPE STANDARD TABLE  ,

                <l_field> TYPE ANY.

     READ TABLE gt_itab ASSIGNING <l_field>. "with table key pernr =  <l_table>-pernr.

LOOP at gt_itab.

check <l_field> is assigned.

ASSIGN <l_field> TO <l_table>. -------- I get runtime this line

ENDLOOP.

Note: My gt_itab (interna table) and <l_field> are loaded.