diff --git a/bookwyrm/activitypub/base_activity.py b/bookwyrm/activitypub/base_activity.py
index 7ee8ca45..933745e0 100644
--- a/bookwyrm/activitypub/base_activity.py
+++ b/bookwyrm/activitypub/base_activity.py
@@ -68,6 +68,8 @@ class ActivityObject:
         for field in fields(self):
             try:
                 value = kwargs[field.name]
+                if value in (None, MISSING):
+                    raise KeyError()
                 try:
                     is_subclass = issubclass(field.type, ActivityObject)
                 except TypeError: