diff --git a/changelog.d/17727.bugfix b/changelog.d/17727.bugfix
new file mode 100644
index 00000000000..64c6e90d87e
--- /dev/null
+++ b/changelog.d/17727.bugfix
@@ -0,0 +1 @@
+Fix a bug in SSS which could prevent /sync from working for certain user accounts.
diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py
index 94a7efee735..03b4aa33818 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -1469,6 +1469,10 @@ async def bulk_get_last_event_pos_in_room_before_stream_ordering(
         recheck_rooms: Set[str] = set()
         min_token = end_token.stream
         for room_id, stream in uncapped_results.items():
+            if stream is None:
+                # Despite the function not directly setting None, the cache can!
+                # See: https://github.com/element-hq/synapse/issues/17726
+                continue
             if stream <= min_token:
                 results[room_id] = stream
             else:
@@ -1495,7 +1499,7 @@ async def _get_max_event_pos(self, room_id: str) -> int:
     @cachedList(cached_method_name="_get_max_event_pos", list_name="room_ids")
     async def _bulk_get_max_event_pos(
         self, room_ids: StrCollection
-    ) -> Mapping[str, int]:
+    ) -> Mapping[str, Optional[int]]:
         """Fetch the max position of a persisted event in the room."""
 
         # We need to be careful not to return positions ahead of the current