A PHP Error was encountered in CodeIgniter -

help:

i have error: 

a php error encountered

severity: warning

message: illegal string offset 'periode'

filename: models/k3m.php

line number: 32

a php error encountered

severity: warning

message: illegal string offset 'bulan'

filename: models/k3m.php

line number: 33

fatal error: call member function row() on non-object in c:\laporbos\htdocs\laporbos3\application\modules\laporan_k3\models\k3m.php on line 39

    function saldo_bank($param) {     $periode = $param['periode'];     $bulan = $param['bulan'];     $bulan = $this->get_bulan2($periode,$bulan);     $sql="select ifnull( sum(ifnull(jumlah_debet,0) ),0) - ifnull( sum(ifnull(jumlah_kredit,0) ),0) saldo             buku_bank             year(tanggal) =".$this->session->userdata("tahun_anggaran")."              , month(tanggal) < $bulan";     $data = $this->db->query($sql)->row();     return $data->saldo; } 

this occurring because calling row() before have called result(), change this

$data = $this->db->query($sql)->row(); 

to this

$data = $this->db->query($sql)->result()->row();