ORA-06502:字符串缓冲区太小。即使字符串大小低于声明的大小限制

 FOR this_loop
     IN (SELECT field_A, field_B
           FROM TABLE_NAME
          WHERE    num = i_num)
  LOOP
     IF this_loop.field_B BETWEEN 1 AND 3
     THEN
        v_A :=  v_A || ' ' || this_loop.field_A;
     ELSIF this_loop.field_B BETWEEN 4 AND 8
     THEN
        v_field_A := v_field_A || ' ' || this_loop.field_A;  -- Error is at this line
     ELSIF this_loop.field_B BETWEEN 9 AND 15
     THEN
        v_B :=  v_B || ' ' || this_loop.field_A;
     END IF;
  END LOOP;
 FOR this_loop
     IN (SELECT field_A, field_B