diff --git a/addons/project/models/project.py b/addons/project/models/project.py
index e2ed0cfd83d..136477a4c1e 100644
--- a/addons/project/models/project.py
+++ b/addons/project/models/project.py
@@ -473,7 +473,7 @@ class Task(models.Model):
     def _compute_attachment_ids(self):
         for task in self:
             attachment_ids = self.env['ir.attachment'].search([('res_id', '=', task.id), ('res_model', '=', 'project.task')]).ids
-            message_attachment_ids = self.mapped('message_ids.attachment_ids').ids  # from mail_thread
+            message_attachment_ids = task.mapped('message_ids.attachment_ids').ids  # from mail_thread
             task.attachment_ids = list(set(attachment_ids) - set(message_attachment_ids))
 
     @api.multi