diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 0687c9fa79..f55a73c160 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -22,7 +22,7 @@
 import logging
 import random
 from http import HTTPStatus
-from typing import TYPE_CHECKING, Any, Mapping, Optional, Sequence
+from typing import TYPE_CHECKING, Any, Mapping, Optional, Sequence, cast
 
 from canonicaljson import encode_canonical_json
 
@@ -1871,8 +1871,8 @@ class EventCreationHandler:
 
             state_entry = await self.state.resolve_state_groups_for_events(
                 event.room_id,
-                event_ids=event.prev_state_events
-                if isinstance(event, FrozenEventVMSC4242)
+                event_ids=cast(FrozenEventVMSC4242, event).prev_state_events
+                if event.room_version.msc4242_state_dags
                 else event.prev_event_ids(),
             )
 
diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index 2f0e3f2c3e..40b2ac3c75 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -29,6 +29,7 @@ from typing import (
     Mapping,
     Optional,
     Sequence,
+    cast,
 )
 
 import attr
@@ -314,8 +315,8 @@ class StateHandler:
             # their states - but I don't think that really matters; it just means we
             # might redundantly recalculate the state for this event later.)
             prev_event_ids = frozenset(
-                event.prev_state_events
-                if isinstance(event, FrozenEventVMSC4242)
+                cast(FrozenEventVMSC4242, event).prev_state_events
+                if event.room_version.msc4242_state_dags
                 else event.prev_event_ids()
             )
             incomplete_prev_events = await self.store.get_partial_state_events(