Hi , im new to odoo . Trying to explore payroll module , having difficulty in deducting salary while generating payslips

So far i have created salary rule "Unpaid leave" with code "UL" , its python code is 

result=-(contract.wage / 22)*(worked_days.no_of_days)

I have also approved unpaid leaves for an employee , when i try to compute payslip for that employee for the same month on which the leaves were approved , Unpaid Leave gives me amount zero . Am i missing something ...  Help would be appreciated