diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index c439b5380..189212b0f 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -221,8 +221,9 @@ class EventContext(object):
         """Gets the current state IDs if we have them already cached.
 
         Returns:
-            dict[(str, str), str]|None: Returns None if state_group
-            is None, which happens when the associated event is an outlier.
+            dict[(str, str), str]|None: Returns None if we haven't cached the
+            state or if state_group is None, which happens when the associated
+            event is an outlier.
         """
 
         return self._current_state_ids
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index dc0b3c2eb..c2910094d 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -549,6 +549,9 @@ class EventsStore(EventsWorkerStore):
             if ctx.state_group in state_groups_map:
                 continue
 
+            # We're only interested in pulling out state that has already
+            # been cached in the context. We'll pull stuff out of the DB later
+            # if necessary.
             current_state_ids = ctx.get_cached_current_state_ids()
             if current_state_ids is not None:
                 state_groups_map[ctx.state_group] = current_state_ids