in another project, I used compute field like this on a function and it works. But why this one is not working?

Here is my code :

from openerp import models, fields, api

class PurchaseOrder(models.Model):

_inherit = 'hr.payslip'

gaji_pokok = fields.Integer(string="Gaji Pokok")

t_lembur = fields.Integer(string="Tunjangan Lembur")

t_transportasi = fields.Integer(string="Tunjangan Transportasi")

bonus = fields.Integer(string="Bonus")

jumlah_kerja = fields.Integer(string="Jumlah Jam Kerja")

libur = fields.Integer(string="Cuti")

total_pendapatan = fields.Monetary(compute = '_compute_gaji', readonly=True, string="Total Gaji")

@api.one

@api.depends('gaji_pokok', 't_lembur', 't_transportasi', 'bonus', 'jumlah_kerja', 'libur')

def _compute_gaji(self):

for sal in self:

if sal.gaji_pokok is not False:

sal.total_pendapatan = sal.gaji_pokok + sal.t_lembur + sal.t_transportasi + sal.bonus + sal.jumlah_kerja + sal.libur

Here is my xml (not all the code I show)

<xpath expr="//page[1]" position="after">

<form colspan="4" nolabel="1">

<page string="Pendapatan Karyawan">

<separator string="Pendapatan Karyawan"/>

<group col="4">

<field name="jumlah_kerja"/>

<field name="libur"/>

<field name="gaji_pokok"/>

<field name="t_lembur"/>

<field name="t_transportasi"/>

<field name="bonus"/>

<field name="total_pendapatan"/>

</group>

</page>

</form>

</xpath>

My error message is "field total_pendapatan doesnt exist"

Anyone can help me about this?