diff --git a/addons/hr_expense/models/hr_expense.py b/addons/hr_expense/models/hr_expense.py
index e3e2bc99d2f..07997ac040c 100644
--- a/addons/hr_expense/models/hr_expense.py
+++ b/addons/hr_expense/models/hr_expense.py
@@ -732,12 +732,12 @@ class HrExpenseSheet(models.Model):
             return self.employee_id.parent_id.user_id
         elif self.employee_id.department_id.manager_id.user_id:
             return self.employee_id.department_id.manager_id.user_id
-        return self.env.user
+        return self.env['res.users']
 
     def activity_update(self):
         for expense_report in self.filtered(lambda hol: hol.state == 'submit'):
             self.activity_schedule(
                 'hr_expense.mail_act_expense_approval',
-                user_id=expense_report.sudo()._get_responsible_for_approval().id)
+                user_id=expense_report.sudo()._get_responsible_for_approval().id or self.env.user.id)
         self.filtered(lambda hol: hol.state == 'approve').activity_feedback(['hr_expense.mail_act_expense_approval'])
         self.filtered(lambda hol: hol.state == 'cancel').activity_unlink(['hr_expense.mail_act_expense_approval'])
diff --git a/addons/hr_holidays/models/hr_leave.py b/addons/hr_holidays/models/hr_leave.py
index 4280bd018e6..c2121ace15e 100644
--- a/addons/hr_holidays/models/hr_leave.py
+++ b/addons/hr_holidays/models/hr_leave.py
@@ -793,7 +793,7 @@ class HolidaysRequest(models.Model):
             return self.employee_id.parent_id.user_id
         elif self.department_id.manager_id.user_id:
             return self.department_id.manager_id.user_id
-        return self.env.user
+        return self.env['res.users']
 
     def activity_update(self):
         to_clean, to_do = self.env['hr.leave'], self.env['hr.leave']
@@ -803,12 +803,12 @@ class HolidaysRequest(models.Model):
             elif holiday.state == 'confirm':
                 holiday.activity_schedule(
                     'hr_holidays.mail_act_leave_approval',
-                    user_id=holiday.sudo()._get_responsible_for_approval().id)
+                    user_id=holiday.sudo()._get_responsible_for_approval().id or self.env.user.id)
             elif holiday.state == 'validate1':
                 holiday.activity_feedback(['hr_holidays.mail_act_leave_approval'])
                 holiday.activity_schedule(
                     'hr_holidays.mail_act_leave_second_approval',
-                    user_id=holiday.sudo()._get_responsible_for_approval().id)
+                    user_id=holiday.sudo()._get_responsible_for_approval().id or self.env.user.id)
             elif holiday.state == 'validate':
                 to_do |= holiday
             elif holiday.state == 'refuse':
diff --git a/addons/hr_holidays/models/hr_leave_allocation.py b/addons/hr_holidays/models/hr_leave_allocation.py
index b225e6c9b70..6a589c397a6 100644
--- a/addons/hr_holidays/models/hr_leave_allocation.py
+++ b/addons/hr_holidays/models/hr_leave_allocation.py
@@ -543,7 +543,7 @@ class HolidaysAllocation(models.Model):
             return self.employee_id.parent_id.user_id
         elif self.department_id.manager_id.user_id:
             return self.department_id.manager_id.user_id
-        return self.env.user
+        return self.env['res.users']
 
     def activity_update(self):
         to_clean, to_do = self.env['hr.leave.allocation'], self.env['hr.leave.allocation']
@@ -553,12 +553,12 @@ class HolidaysAllocation(models.Model):
             elif allocation.state == 'confirm':
                 allocation.activity_schedule(
                     'hr_holidays.mail_act_leave_allocation_approval',
-                    user_id=allocation.sudo()._get_responsible_for_approval().id)
+                    user_id=allocation.sudo()._get_responsible_for_approval().id or self.env.user.id)
             elif allocation.state == 'validate1':
                 allocation.activity_feedback(['hr_holidays.mail_act_leave_allocation_approval'])
                 allocation.activity_schedule(
                     'hr_holidays.mail_act_leave_allocation_second_approval',
-                    user_id=allocation.sudo()._get_responsible_for_approval().id)
+                    user_id=allocation.sudo()._get_responsible_for_approval().id or self.env.user.id)
             elif allocation.state == 'validate':
                 to_do |= allocation
             elif allocation.state == 'refuse':