diff --git a/changelog.d/8457.bugfix b/changelog.d/8457.bugfix
new file mode 100644
index 0000000000..545b06d180
--- /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 8c907ad596..56f8dc9caf 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -446,6 +446,8 @@ def check_redaction(
 
     if room_version_obj.event_format == EventFormatVersions.V1:
         redacter_domain = get_domain_from_id(event.event_id)
+        if not isinstance(event.redacts, str):
+            return False
         redactee_domain = get_domain_from_id(event.redacts)
         if redacter_domain == redactee_domain:
             return True