diff --git a/changelog.d/13748.misc b/changelog.d/13748.misc
new file mode 100644
index 000000000000..2f419bb659a1
--- /dev/null
+++ b/changelog.d/13748.misc
@@ -0,0 +1 @@
+Avoid raising an error due to malformed user IDs in `get_current_hosts_in_room`. Malformed user IDs cannot currently join a room, so this error would not be hit.
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index a77e49dc66e3..9c71563662d0 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -1044,6 +1044,8 @@ async def get_current_hosts_in_room(self, room_id: str) -> List[str]:
             # We use a `Set` just for fast lookups
             domain_set: Set[str] = set()
             for u in users:
+                if ":" not in u:
+                    continue
                 domain = get_domain_from_id(u)
                 if domain not in domain_set:
                     domain_set.add(domain)
@@ -1071,13 +1073,15 @@ def get_current_hosts_in_room_txn(txn: LoggingTransaction) -> List[str]:
                     c.type = 'm.room.member'
                     AND c.membership = 'join'
                     AND c.room_id = ?
+                    AND server_domain IS NOT NULL
                 /* Group all state events from the same domain into their own buckets (groups) */
                 GROUP BY server_domain
                 /* Sorted by lowest depth first */
                 ORDER BY min(e.depth) ASC;
             """
             txn.execute(sql, (room_id,))
-            return [d for d, in txn]
+            # `server_domain` will be `NULL` for malformed MXIDs with no colons.
+            return [d for d, in txn if d is not None]
 
         return await self.db_pool.runInteraction(
             "get_current_hosts_in_room", get_current_hosts_in_room_txn