diff --git a/addons/project/models/project.py b/addons/project/models/project.py
index 11f1c838d50..ce9c41be45f 100644
--- a/addons/project/models/project.py
+++ b/addons/project/models/project.py
@@ -454,6 +454,12 @@ class Task(models.Model):
             return False
         return self.stage_find(project_id, [('fold', '=', False)])
 
+    @api.model
+    def _default_company_id(self):
+        if self._context.get('default_project_id'):
+            return self.env['project.project'].browse(self._context['default_project_id']).company_id
+        return self.env.company
+
     @api.model
     def _read_group_stage_ids(self, stages, domain, order):
         search_domain = [('id', 'in', stages.ids)]
@@ -508,7 +514,7 @@ class Task(models.Model):
         default=lambda self: self._get_default_partner(),
         domain="['|', ('company_id', '=', False), ('company_id', '=', company_id)]")
     manager_id = fields.Many2one('res.users', string='Project Manager', related='project_id.user_id', readonly=True, related_sudo=False)
-    company_id = fields.Many2one('res.company', string='Company', required=True, default=lambda self: self.env.company)
+    company_id = fields.Many2one('res.company', string='Company', required=True, default=_default_company_id)
     color = fields.Integer(string='Color Index')
     user_email = fields.Char(related='user_id.email', string='User Email', readonly=True, related_sudo=False)
     attachment_ids = fields.One2many('ir.attachment', compute='_compute_attachment_ids', string="Main Attachments",
@@ -616,6 +622,8 @@ class Task(models.Model):
                 self.partner_id = self.project_id.partner_id
             if self.project_id not in self.stage_id.project_ids:
                 self.stage_id = self.stage_find(self.project_id.id, [('fold', '=', False)])
+            # keep multi company consistency
+            self.company_id = self.project_id.company_id
         else:
             self.stage_id = False
 
@@ -625,6 +633,13 @@ class Task(models.Model):
             if task.parent_id and task.child_ids:
                 raise ValidationError(_('Task %s cannot have several subtask levels.' % (task.name,)))
 
+    @api.constrains('company_id', 'project_id')
+    def _check_multi_company(self):
+        for task in self:
+            if task.project_id:
+                if task.project_id.company_id != task.company_id:
+                    raise ValidationError(_('Your task must beo in the same company as its project.'))
+
     @api.returns('self', lambda value: value.id)
     def copy(self, default=None):
         if default is None:
diff --git a/addons/sale_timesheet/models/project.py b/addons/sale_timesheet/models/project.py
index 17c80bf21b2..ed54d749a3c 100644
--- a/addons/sale_timesheet/models/project.py
+++ b/addons/sale_timesheet/models/project.py
@@ -183,7 +183,8 @@ class ProjectTask(models.Model):
     @api.onchange('project_id')
     def _onchange_project(self):
         result = super(ProjectTask, self)._onchange_project()
-        self.sale_line_id = self.project_id.sale_line_id
+        if self.project_id.sale_line_id:
+            self.sale_line_id = self.project_id.sale_line_id
         if not self.parent_id and not self.partner_id:
             self.partner_id = self.sale_line_id.order_partner_id
         # set domain on SO: on non billable project, all SOL of customer, otherwise the one from the SO