diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 8852db4eaf..5969a7130f 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -664,10 +664,6 @@ class EventCreationHandler:
                 500, "Tried to send member event through non-member codepath"
             )
 
-        user = UserID.from_string(event.sender)
-
-        assert self.hs.is_mine(user), "User must be our own: %s" % (user,)
-
         ev = await self.handle_new_client_event(
             requester=requester,
             event=event,
@@ -748,6 +744,10 @@ class EventCreationHandler:
                 requester, event_dict, token_id=requester.access_token_id, txn_id=txn_id
             )
 
+            assert self.hs.is_mine_id(event.sender), "User must be our own: %s" % (
+                event.sender,
+            )
+
             spam_error = self.spam_checker.check_event_for_spam(event)
             if spam_error:
                 if not isinstance(spam_error, str):
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index f14f791586..530bf0ab88 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -185,6 +185,7 @@ class RoomCreationHandler(BaseHandler):
             ShadowBanError if the requester is shadow-banned.
         """
         user_id = requester.user.to_string()
+        assert self.hs.is_mine_id(user_id), "User must be our own: %s" % (user_id,)
 
         # start by allocating a new room id
         r = await self.store.get_room(old_room_id)