diff --git a/taiga/projects/history/freeze_impl.py b/taiga/projects/history/freeze_impl.py
index ae7eec7c..85c47d4f 100644
--- a/taiga/projects/history/freeze_impl.py
+++ b/taiga/projects/history/freeze_impl.py
@@ -93,7 +93,8 @@ def _common_users_values(diff):
     if "assigned_to" in diff:
         users.update(diff["assigned_to"])
     if "assigned_users" in diff:
-        [users.update(usrs_id) if usrs_id else None for usrs_id in diff["assigned_users"]]
+        [users.update(usrs_ids) for usrs_ids in diff["assigned_users"] if
+         usrs_ids]
     if users:
         values["users"] = _get_users_values(users)
 
@@ -335,6 +336,12 @@ def userstory_freezer(us) -> dict:
     for rp in rpqsd:
         points[str(rp.role_id)] = rp.points_id
 
+    assigned_users = [u.id for u in us.assigned_users.all()]
+    # Due to multiple assignment migration, for new snapshots we add to
+    # assigned users a list with the 'assigned to' value
+    if us.assigned_to_id and not assigned_users:
+        assigned_users = [us.assigned_to_id]
+
     snapshot = {
         "ref": us.ref,
         "owner": us.owner_id,
@@ -348,7 +355,7 @@ def userstory_freezer(us) -> dict:
         "description": us.description,
         "description_html": mdrender(us.project, us.description),
         "assigned_to": us.assigned_to_id,
-        "assigned_users": [u.id for u in us.assigned_users.all()],
+        "assigned_users": assigned_users,
         "milestone": us.milestone_id,
         "client_requirement": us.client_requirement,
         "team_requirement": us.team_requirement,
diff --git a/taiga/projects/history/services.py b/taiga/projects/history/services.py
index ef684f1e..4f3487fc 100644
--- a/taiga/projects/history/services.py
+++ b/taiga/projects/history/services.py
@@ -229,6 +229,31 @@ def get_excluded_fields(typename: str) -> tuple:
     return _deprecated_fields.get(typename, ())
 
 
+def migrate_userstory_diff(obj: FrozenObj) -> FrozenObj:
+    # Due to multiple assignment migration, for old snapshots we add a list
+    # with the 'assigned to' value
+    if 'assigned_users' not in obj.snapshot.keys():
+        snapshot = deepcopy(obj.snapshot)
+        snapshot['assigned_users'] = [obj.snapshot['assigned_to']]
+
+        obj = FrozenObj(obj.key, snapshot)
+
+    return obj
+
+
+_migrations = {"userstories.userstory": migrate_userstory_diff}
+
+
+def migrate_to_last_version(typename: str, obj: FrozenObj) -> FrozenObj:
+    """""
+    Adapt old snapshots to the last format in order to generate correct diffs.
+    :param typename:
+    :param obj:
+    :return:
+    """
+    return _migrations.get(typename, lambda x: x)(obj)
+
+
 def make_diff(oldobj: FrozenObj, newobj: FrozenObj,
               excluded_keys: tuple = ()) -> FrozenDiff:
     """
@@ -342,6 +367,10 @@ def take_snapshot(obj: object, *, comment: str="", user=None,
         new_fobj = freeze_model_instance(obj)
         old_fobj, need_real_snapshot = get_last_snapshot_for_key(key)
 
+        # migrate diff to latest schema
+        if old_fobj:
+            old_fobj = migrate_to_last_version(typename, old_fobj)
+
         entry_model = apps.get_model("history", "HistoryEntry")
         user_id = None if user is None else user.id
         user_name = "" if user is None else user.get_full_name()
@@ -358,6 +387,7 @@ def take_snapshot(obj: object, *, comment: str="", user=None,
             raise RuntimeError("Unexpected condition")
 
         excluded_fields = get_excluded_fields(typename)
+
         fdiff = make_diff(old_fobj, new_fobj, excluded_fields)
 
         # If diff and comment are empty, do
diff --git a/taiga/projects/userstories/serializers.py b/taiga/projects/userstories/serializers.py
index 3177884b..2e34fff4 100644
--- a/taiga/projects/userstories/serializers.py
+++ b/taiga/projects/userstories/serializers.py
@@ -90,7 +90,16 @@ class UserStoryListSerializer(ProjectExtraInfoSerializerMixin,
 
         :return: User queryset object representing the assigned users
         """
-        return [user.id for user in obj.assigned_users.all()]
+        if not obj.assigned_to:
+            return set([user.id for user in obj.assigned_users.all()])
+
+        assigned_users = [user.id for user in obj.assigned_users.all()] + \
+                         [obj.assigned_to.id]
+
+        if not assigned_users:
+            return None
+
+        return set(assigned_users)
 
     def get_epic_order(self, obj):
         include_epic_order = getattr(obj, "include_epic_order", False)
diff --git a/tests/integration/test_userstories.py b/tests/integration/test_userstories.py
index 87c87947..82979b89 100644
--- a/tests/integration/test_userstories.py
+++ b/tests/integration/test_userstories.py
@@ -104,7 +104,7 @@ def test_create_userstory_with_assigned_users(client):
     response = client.json.post(url, json_data)
 
     assert response.status_code == 201
-    assert response.data["assigned_users"] == [user.id, user_watcher.id]
+    assert response.data["assigned_users"] == set([user.id, user_watcher.id])
 
 
 def test_create_userstory_with_watchers(client):