diff --git a/changelog.d/8110.bugfix b/changelog.d/8110.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..5269a232e107643352ea07672d6bbd407d906c11
--- /dev/null
+++ b/changelog.d/8110.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.12.0 which could cause `/sync` requests to fail with a 404 if you had a very old outstanding room invite.
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index cc5deca75bf091526786095601b087e836d0d69b..67db763dbfd193341c305e724d5f636a163f0499 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -133,6 +133,8 @@ class _EventInternalMetadata(object):
         rejection. This is needed as those events are marked as outliers, but
         they still need to be processed as if they're new events (e.g. updating
         invite state in the database, relaying to clients, etc).
+
+        (Added in synapse 0.99.0, so may be unreliable for events received before that)
         """
         return self._dict.get("out_of_band_membership", False)
 
diff --git a/synapse/storage/databases/main/events_worker.py b/synapse/storage/databases/main/events_worker.py
index 4a3333c0db4a9835d02f81103d3839e87b008566..e1241a724b675feefc3361fab49fd1b54d6b7f1e 100644
--- a/synapse/storage/databases/main/events_worker.py
+++ b/synapse/storage/databases/main/events_worker.py
@@ -620,19 +620,38 @@ class EventsWorkerStore(SQLBaseStore):
             room_version_id = row["room_version_id"]
 
             if not room_version_id:
-                # this should only happen for out-of-band membership events
-                if not internal_metadata.get("out_of_band_membership"):
-                    logger.warning(
-                        "Room %s for event %s is unknown", d["room_id"], event_id
+                # this should only happen for out-of-band membership events which
+                # arrived before #6983 landed. For all other events, we should have
+                # an entry in the 'rooms' table.
+                #
+                # However, the 'out_of_band_membership' flag is unreliable for older
+                # invites, so just accept it for all membership events.
+                #
+                if d["type"] != EventTypes.Member:
+                    raise Exception(
+                        "Room %s for event %s is unknown" % (d["room_id"], event_id)
                     )
-                    continue
 
-                # take a wild stab at the room version based on the event format
+                # so, assuming this is an out-of-band-invite that arrived before #6983
+                # landed, we know that the room version must be v5 or earlier (because
+                # v6 hadn't been invented at that point, so invites from such rooms
+                # would have been rejected.)
+                #
+                # The main reason we need to know the room version here (other than
+                # choosing the right python Event class) is in case the event later has
+                # to be redacted - and all the room versions up to v5 used the same
+                # redaction algorithm.
+                #
+                # So, the following approximations should be adequate.
+
                 if format_version == EventFormatVersions.V1:
+                    # if it's event format v1 then it must be room v1 or v2
                     room_version = RoomVersions.V1
                 elif format_version == EventFormatVersions.V2:
+                    # if it's event format v2 then it must be room v3
                     room_version = RoomVersions.V3
                 else:
+                    # if it's event format v3 then it must be room v4 or v5
                     room_version = RoomVersions.V5
             else:
                 room_version = KNOWN_ROOM_VERSIONS.get(room_version_id)