call custom field in function or bean

hi,
i have a problem,
i don’t know how to call the custom field in a function or bean.
this is the example of my code


public function sumTotalFromEachRow($bean, $event, $arguments)
    {
        self::$sum += $bean->biaya_bulanan_c;
    }

the result is zero, whats wrong with my code?
i tested with a field amount_usdollar in Opportunities just like this



public function sumTotalFromEachRow($bean, $event, $arguments)
    {
        self::$sum += $bean->amount_usdollar;
    }


and its works. but why if the custom code doesnt work?

thankyou
regards
agas

need help

I think it is like this

${bean_cstm->Field_c}	References a custom field of the current report module	${bean_cstm->account_type_c}

Info taken from Asol Reports Documentation

Best Regards