diff --git a/event/models.py b/event/models.py
index 66db679..957e13a 100644
--- a/event/models.py
+++ b/event/models.py
@@ -167,6 +167,7 @@ class Activity(AbstractActivityTemplate):
                     ActivityTemplate,
                     related_name="children",
                     blank=True,
+                    null=True,
                 )
     staff = models.ManyToManyField(
                     User,