diff --git a/changelog.d/8457.bugfix b/changelog.d/8457.bugfix
new file mode 100644
index 000000000000..545b06d180c1
--- /dev/null
+++ b/changelog.d/8457.bugfix
@@ -0,0 +1 @@
+Fix a bug where backfilling a room with an event that was missing the `redacts` field would break.
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index 8c907ad5969a..0b67a59118ac 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -446,7 +446,9 @@ def check_redaction(
 
     if room_version_obj.event_format == EventFormatVersions.V1:
         redacter_domain = get_domain_from_id(event.event_id)
-        redactee_domain = get_domain_from_id(event.redacts)
+        redactee_domain = None
+        if event.redacts:
+            redactee_domain = get_domain_from_id(event.redacts)
         if redacter_domain == redactee_domain:
             return True
     else: