diff --git a/changelog.d/19768.misc b/changelog.d/19768.misc
new file mode 100644
index 00000000000..8314491ced0
--- /dev/null
+++ b/changelog.d/19768.misc
@@ -0,0 +1 @@
+Allow the Third Party Rules `check_event_allowed()` callback to work with [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291) rooms. Contributed by @famedly.
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 0687c9fa790..17d6c49ce3d 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -2376,7 +2376,7 @@ async def _rebuild_event_after_third_party_rules(
                 original_event.room_version, third_party_result
             )
             self.validator.validate_builder(builder)
-            assert builder.room_id is not None
+
         except SynapseError as e:
             raise Exception(
                 "Third party rules module created an invalid event: " + e.msg,
@@ -2411,12 +2411,20 @@ async def _rebuild_event_after_third_party_rules(
         for k, v in original_event.internal_metadata.get_dict().items():
             setattr(builder.internal_metadata, k, v)
 
-        # modules can send new state events, so we re-calculate the auth events just in
-        # case.
-        prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
+        # Creation events using msc4291 rooms will not have a room_id, and will
+        # also not have prev_events nor prev_state_events(below). This makes mypy happy.
+        prev_event_ids = []
+        if builder.room_id is not None:
+            prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
 
         prev_state_events = None
-        if original_event.room_version.msc4242_state_dags:
+        if (
+            original_event.room_version.msc4242_state_dags
+            and builder.type != EventTypes.Create
+            and builder.room_id is not None
+        ):
+            # modules can send new state events, so we re-calculate the auth events just
+            # in case.
             prev_state_events = list(
                 await self.store.get_state_dag_extremities(builder.room_id)
             )
diff --git a/tests/rest/client/test_third_party_rules.py b/tests/rest/client/test_third_party_rules.py
index 1709b27f67c..95afc5e82b6 100644
--- a/tests/rest/client/test_third_party_rules.py
+++ b/tests/rest/client/test_third_party_rules.py
@@ -382,7 +382,8 @@ def test_send_event(self) -> None:
             "third_party_event_rules": {
                 "module": __name__ + ".LegacyChangeEvents",
                 "config": {},
-            }
+            },
+            "default_room_version": "12",
         }
     )
     def test_legacy_check_event_allowed(self) -> None: