diff --git a/changelog.d/19235.bugfix b/changelog.d/19235.bugfix
new file mode 100644
index 00000000000..1c312351a41
--- /dev/null
+++ b/changelog.d/19235.bugfix
@@ -0,0 +1 @@
+Fix v12 rooms when running with `use_frozen_dicts: True`.
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 5f786037824..c7eaf7eda2b 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -548,7 +548,7 @@ def auth_event_ids(self) -> StrCollection:
         assert create_event_id not in self._dict["auth_events"]
         if self.type == EventTypes.Create and self.get_state_key() == "":
             return self._dict["auth_events"]  # should be []
-        return self._dict["auth_events"] + [create_event_id]
+        return [*self._dict["auth_events"], create_event_id]
 
 
 def _event_type_from_format_version(
diff --git a/tests/federation/test_federation_server.py b/tests/federation/test_federation_server.py
index 0d747912906..c4491d5b3c9 100644
--- a/tests/federation/test_federation_server.py
+++ b/tests/federation/test_federation_server.py
@@ -30,6 +30,7 @@
 from synapse.api.errors import FederationError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersions
 from synapse.config.server import DEFAULT_ROOM_VERSION
+from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.events import EventBase, make_event_from_dict
 from synapse.federation.federation_base import event_from_pdu_json
 from synapse.http.types import QueryParams
@@ -356,19 +357,44 @@ def _make_join(self, user_id: str) -> JsonDict:
         self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
         return channel.json_body
 
-    def test_send_join(self) -> None:
+    def _test_send_join_common(self, room_version: str) -> None:
         """happy-path test of send_join"""
+        creator_user_id = self.register_user(f"kermit_v{room_version}", "test")
+        tok = self.login(f"kermit_v{room_version}", "test")
+        room_id = self.helper.create_room_as(
+            room_creator=creator_user_id, tok=tok, room_version=room_version
+        )
+
+        # Second member joins
+        second_member_user_id = self.register_user(f"fozzie_v{room_version}", "bear")
+        tok2 = self.login(f"fozzie_v{room_version}", "bear")
+        self.helper.join(room_id, second_member_user_id, tok=tok2)
+
+        # Make join for remote user
         joining_user = "@misspiggy:" + self.OTHER_SERVER_NAME
-        join_result = self._make_join(joining_user)
+        channel = self.make_signed_federation_request(
+            "GET",
+            f"/_matrix/federation/v1/make_join/{room_id}/{joining_user}?ver={room_version}",
+        )
+        self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
+        join_result = channel.json_body
 
+        # Sign and send the join
         join_event_dict = join_result["event"]
         self.add_hashes_and_signatures_from_other_server(
             join_event_dict,
-            KNOWN_ROOM_VERSIONS[DEFAULT_ROOM_VERSION],
+            KNOWN_ROOM_VERSIONS[room_version],
         )
+        if room_version in ["1", "2"]:
+            add_hashes_and_signatures(
+                KNOWN_ROOM_VERSIONS[room_version],
+                join_event_dict,
+                signature_name=self.hs.hostname,
+                signing_key=self.hs.signing_key,
+            )
         channel = self.make_signed_federation_request(
             "PUT",
-            f"/_matrix/federation/v2/send_join/{self._room_id}/x",
+            f"/_matrix/federation/v2/send_join/{room_id}/x",
             content=join_event_dict,
         )
         self.assertEqual(channel.code, HTTPStatus.OK, channel.json_body)
@@ -384,8 +410,8 @@ def test_send_join(self) -> None:
                 ("m.room.power_levels", ""),
                 ("m.room.join_rules", ""),
                 ("m.room.history_visibility", ""),
-                ("m.room.member", "@kermit:test"),
-                ("m.room.member", "@fozzie:test"),
+                ("m.room.member", f"@kermit_v{room_version}:test"),
+                ("m.room.member", f"@fozzie_v{room_version}:test"),
                 # nb: *not* the joining user
             ],
         )
@@ -398,18 +424,28 @@ def test_send_join(self) -> None:
             returned_auth_chain_events,
             [
                 ("m.room.create", ""),
-                ("m.room.member", "@kermit:test"),
+                ("m.room.member", f"@kermit_v{room_version}:test"),
                 ("m.room.power_levels", ""),
                 ("m.room.join_rules", ""),
             ],
         )
 
         # the room should show that the new user is a member
-        r = self.get_success(
-            self._storage_controllers.state.get_current_state(self._room_id)
-        )
+        r = self.get_success(self._storage_controllers.state.get_current_state(room_id))
         self.assertEqual(r[("m.room.member", joining_user)].membership, "join")
 
+    @parameterized.expand([(k,) for k in KNOWN_ROOM_VERSIONS.keys()])
+    @override_config({"use_frozen_dicts": True})
+    def test_send_join_with_frozen_dicts(self, room_version: str) -> None:
+        """Test send_join with USE_FROZEN_DICTS=True"""
+        self._test_send_join_common(room_version)
+
+    @parameterized.expand([(k,) for k in KNOWN_ROOM_VERSIONS.keys()])
+    @override_config({"use_frozen_dicts": False})
+    def test_send_join_without_frozen_dicts(self, room_version: str) -> None:
+        """Test send_join with USE_FROZEN_DICTS=False"""
+        self._test_send_join_common(room_version)
+
     def test_send_join_partial_state(self) -> None:
         """/send_join should return partial state, if requested"""
         joining_user = "@misspiggy:" + self.OTHER_SERVER_NAME