**Impacted versions:**
11.0

**Steps to reproduce:**
In runbot odoo/odoo or odoo/enterprise

- In Preferences change the timezone to America/Mexico_city

- Create a contract for a user with this values: **state**: running, **End Date**: NULL, **Start Date**: year before (for example), and **working schedule**: Standard 40 Hours/week
![image](https://user-images.githubusercontent.com/19410285/35942162-2640db56-0c2b-11e8-8714-b25413d62123.png)

- Edit the working schedule as the image below, and save.
![image](https://user-images.githubusercontent.com/19410285/35942368-c846cf82-0c2b-11e8-9a26-753ded2509d1.png)

- Create a payslip for a user with the edited contract.

- Change the date_from (in period) to a saturday day
![image](https://user-images.githubusercontent.com/19410285/35942575-83626d08-0c2c-11e8-8023-11675d29c3ea.png)


**Current behavior:**
Take this error
![image](https://user-images.githubusercontent.com/19410285/35942631-ad048268-0c2c-11e8-9147-c3f2ff5770fe.png)


**Expected behavior:**
compute the number of days without error


**Video/Screenshot link (optional):**
https://youtu.be/fQWTg7LXD2g

else:
            if contract_id:
                
                contract_ids = [contract_id]
            else:
                
                contract_ids = self.get_contract(employee, date_from, date_to)
        if not contract_ids:
            return res
        contract = self.env['hr.contract'].browse(contract_ids[0])
        res['value'].update({
            'contract_id': contract.id
        })
        struct = contract.struct_id
        if not struct:
            return res
        res['value'].update({
            'struct_id': struct.id,
        })
        
        contracts = self.env['hr.contract'].browse(contract_ids)
        worked_days_line_ids = self.get_worked_day_lines(contracts, date_from, date_to)
        input_line_ids = self.get_inputs(contracts, date_from, date_to)
        res['value'].update({
            'worked_days_line_ids': worked_days_line_ids,
            'input_line_ids': input_line_ids,
        })
        return res
    @api.onchange('employee_id', 'date_from', 'date_to')
    def onchange_employee(self):