diff --git a/taiga/projects/notifications/mixins.py b/taiga/projects/notifications/mixins.py
index 27a93d88..03082f3b 100644
--- a/taiga/projects/notifications/mixins.py
+++ b/taiga/projects/notifications/mixins.py
@@ -190,13 +190,14 @@ class WatchedResourceModelSerializer(serializers.ModelSerializer):
         #If that's the case we need to remove it before calling the super method
         watcher_field = self.fields.pop("watchers", None)
         self.validate_watchers(attrs, "watchers")
-        new_watcher_ids = set(attrs.pop("watchers", []))
+        new_watcher_ids = attrs.pop("watchers", None)
         obj = super(WatchedResourceModelSerializer, self).restore_object(attrs, instance)
 
         #A partial update can exclude the watchers field or if the new instance can still not be saved
-        if instance is None or len(new_watcher_ids) == 0:
+        if instance is None or new_watcher_ids is None:
             return obj
 
+        new_watcher_ids = set(new_watcher_ids)
         old_watcher_ids = set(obj.get_watchers().values_list("id", flat=True))
         adding_watcher_ids = list(new_watcher_ids.difference(old_watcher_ids))
         removing_watcher_ids = list(old_watcher_ids.difference(new_watcher_ids))
diff --git a/tests/integration/test_userstories.py b/tests/integration/test_userstories.py
index 787656e8..6506f70f 100644
--- a/tests/integration/test_userstories.py
+++ b/tests/integration/test_userstories.py
@@ -477,3 +477,40 @@ def test_update_userstory_respecting_watchers(client):
     assert response.status_code == 200
     assert response.data["subject"] == "Updating test"
     assert response.data["watchers"] == [watching_user.id]
+
+
+def test_update_userstory_update_watchers(client):
+    watching_user = f.create_user()
+    project = f.ProjectFactory.create()
+    us = f.UserStoryFactory.create(project=project, status__project=project, milestone__project=project)
+    f.MembershipFactory.create(project=us.project, user=us.owner, is_owner=True)
+    f.MembershipFactory.create(project=us.project, user=watching_user)
+
+    client.login(user=us.owner)
+    url = reverse("userstories-detail", kwargs={"pk": us.pk})
+    data = {"watchers": [watching_user.id], "version":1}
+
+    response = client.json.patch(url, json.dumps(data))
+    assert response.status_code == 200
+    assert response.data["watchers"] == [watching_user.id]
+    watcher_ids = list(us.get_watchers().values_list("id", flat=True))
+    assert watcher_ids == [watching_user.id]
+
+
+def test_update_userstory_remove_watchers(client):
+    watching_user = f.create_user()
+    project = f.ProjectFactory.create()
+    us = f.UserStoryFactory.create(project=project, status__project=project, milestone__project=project)
+    us.add_watcher(watching_user)
+    f.MembershipFactory.create(project=us.project, user=us.owner, is_owner=True)
+    f.MembershipFactory.create(project=us.project, user=watching_user)
+
+    client.login(user=us.owner)
+    url = reverse("userstories-detail", kwargs={"pk": us.pk})
+    data = {"watchers": [], "version":1}
+
+    response = client.json.patch(url, json.dumps(data))
+    assert response.status_code == 200
+    assert response.data["watchers"] == []
+    watcher_ids = list(us.get_watchers().values_list("id", flat=True))
+    assert watcher_ids == []