diff --git a/changelog.d/17733.bugfix b/changelog.d/17733.bugfix
new file mode 100644
index 000000000..64c6e90d8
--- /dev/null
+++ b/changelog.d/17733.bugfix
@@ -0,0 +1 @@
+Fix a bug in SSS which could prevent /sync from working for certain user accounts.
diff --git a/synapse/handlers/sliding_sync/room_lists.py b/synapse/handlers/sliding_sync/room_lists.py
index a423de74b..353c491f7 100644
--- a/synapse/handlers/sliding_sync/room_lists.py
+++ b/synapse/handlers/sliding_sync/room_lists.py
@@ -40,6 +40,7 @@ from synapse.api.constants import (
     EventTypes,
     Membership,
 )
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.events import StrippedStateEvent
 from synapse.events.utils import parse_stripped_state_event
 from synapse.logging.opentracing import start_active_span, trace
@@ -952,6 +953,15 @@ class SlidingSyncRoomLists:
             excluded_rooms=self.rooms_to_exclude_globally,
         )
 
+        # We filter out unknown room versions before we try and load any
+        # metadata about the room. They shouldn't go down sync anyway, and their
+        # metadata may be in a broken state.
+        room_for_user_list = [
+            room_for_user
+            for room_for_user in room_for_user_list
+            if room_for_user.room_version_id in KNOWN_ROOM_VERSIONS
+        ]
+
         # Remove invites from ignored users
         ignored_users = await self.store.ignored_users(user_id)
         if ignored_users:
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index e321a1add..ded794871 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -41,6 +41,7 @@ import attr
 
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import Codes, SynapseError
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.logging.opentracing import trace
 from synapse.metrics import LaterGauge
 from synapse.metrics.background_process_metrics import wrap_as_background_process
@@ -1443,6 +1444,10 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
                     is_encrypted=bool(row[9]),
                 )
                 for row in txn
+                # We filter out unknown room versions proactively. They
+                # shouldn't go down sync and their metadata may be in a broken
+                # state (causing errors).
+                if row[4] in KNOWN_ROOM_VERSIONS
             }
 
         return await self.db_pool.runInteraction(
diff --git a/tests/rest/client/sliding_sync/test_rooms_meta.py b/tests/rest/client/sliding_sync/test_rooms_meta.py
index 8d6039715..c619dd83f 100644
--- a/tests/rest/client/sliding_sync/test_rooms_meta.py
+++ b/tests/rest/client/sliding_sync/test_rooms_meta.py
@@ -1198,3 +1198,55 @@ class SlidingSyncRoomsMetaTestCase(SlidingSyncBase):
                 joined_dm_room_id: True,
             },
         )
+
+    def test_old_room_with_unknown_room_version(self) -> None:
+        """Test that an old room with unknown room version does not break
+        sync."""
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # We first create a standard room, then we'll change the room version in
+        # the DB.
+        room_id = self.helper.create_room_as(
+            user1_id,
+            tok=user1_tok,
+        )
+
+        # Poke the database and update the room version to an unknown one.
+        self.get_success(
+            self.hs.get_datastores().main.db_pool.simple_update(
+                "rooms",
+                keyvalues={"room_id": room_id},
+                updatevalues={"room_version": "unknown-room-version"},
+                desc="updated-room-version",
+            )
+        )
+
+        # Invalidate method so that it returns the currently updated version
+        # instead of the cached version.
+        self.hs.get_datastores().main.get_room_version_id.invalidate((room_id,))
+
+        # For old unknown room versions we won't have an entry in this table
+        # (due to us skipping unknown room versions in the background update).
+        self.get_success(
+            self.store.db_pool.simple_delete(
+                table="sliding_sync_joined_rooms",
+                keyvalues={"room_id": room_id},
+                desc="delete_sliding_room",
+            )
+        )
+
+        # Also invalidate some caches to ensure we pull things from the DB.
+        self.store._events_stream_cache._entity_to_key.pop(room_id)
+        self.store._get_max_event_pos.invalidate((room_id,))
+
+        sync_body = {
+            "lists": {
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 5,
+                }
+            }
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)