diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index 9adedbbb02..9199a6055f 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -20,7 +20,13 @@ from signedjson.key import decode_verify_key_bytes
 from signedjson.sign import SignatureVerifyException, verify_signed_json
 from unpaddedbase64 import decode_base64
 
-from synapse.api.constants import KNOWN_ROOM_VERSIONS, EventTypes, JoinRules, Membership
+from synapse.api.constants import (
+    KNOWN_ROOM_VERSIONS,
+    EventTypes,
+    JoinRules,
+    Membership,
+    RoomVersions,
+)
 from synapse.api.errors import AuthError, EventSizeError, SynapseError
 from synapse.types import UserID, get_domain_from_id
 
@@ -49,7 +55,6 @@ def check(room_version, event, auth_events, do_sig_check=True, do_size_check=Tru
 
     if do_sig_check:
         sender_domain = get_domain_from_id(event.sender)
-        event_id_domain = get_domain_from_id(event.event_id)
 
         is_invite_via_3pid = (
             event.type == EventTypes.Member
@@ -66,9 +71,13 @@ def check(room_version, event, auth_events, do_sig_check=True, do_size_check=Tru
             if not is_invite_via_3pid:
                 raise AuthError(403, "Event not signed by sender's server")
 
-        # Check the event_id's domain has signed the event
-        if not event.signatures.get(event_id_domain):
-            raise AuthError(403, "Event not signed by sending server")
+        if event.format_version in (RoomVersions.V1, RoomVersions.V2):
+            # Only older room versions have event IDs to check.
+            event_id_domain = get_domain_from_id(event.event_id)
+
+            # Check the origin domain has signed the event
+            if not event.signatures.get(event_id_domain):
+                raise AuthError(403, "Event not signed by sending server")
 
     if auth_events is None:
         # Oh, we don't know what the state of the room was, so we