diff --git a/addons/project/models/project_task.py b/addons/project/models/project_task.py
index 54776a346e1..9e1c91f16a2 100644
--- a/addons/project/models/project_task.py
+++ b/addons/project/models/project_task.py
@@ -1027,7 +1027,7 @@ class Task(models.Model):
             self = self.sudo()
 
         # Track user_ids to send assignment notifications
-        old_user_ids = {t: t.user_ids for t in self}
+        old_user_ids = {t: t.user_ids for t in self.sudo()}
 
         if "personal_stage_type_id" in vals and not vals['personal_stage_type_id']:
             del vals['personal_stage_type_id']