diff --git a/lib/sync.js b/lib/sync.js
index e7299a39b..cb381f0d7 100644
--- a/lib/sync.js
+++ b/lib/sync.js
@@ -649,6 +649,8 @@ SyncApi.prototype._getGuestFilter = function() {
     if (!guestRooms) {
         return "{}";
     }
+    // we just need to specify the filter inline if we're a guest because guests
+    // can't create filters.
     return JSON.stringify({
         room: {
             timeline: {