diff --git a/sync3/lists.go b/sync3/lists.go
index fc773b0..555c077 100644
--- a/sync3/lists.go
+++ b/sync3/lists.go
@@ -156,6 +156,18 @@ func (s *InternalRequestLists) Get(listKey string) *FilteredSortableRooms {
 	return s.lists[listKey]
 }
 
+// ListKeys returns a copy of the list keys currently tracked by this
+// InternalRequestLists struct, in no particular order. Outside of test code, you
+// probably don't want to call this---you probably have the set of list keys tracked
+// elsewhere in the application.
+func (s *InternalRequestLists) ListKeys() []string {
+	keys := make([]string, len(s.lists))
+	for listKey, _ := range s.lists {
+		keys = append(keys, listKey)
+	}
+	return keys
+}
+
 // ListsByVisibleRoomIDs builds a map from room IDs to a slice of list names. Keys are
 // all room IDs that are currently visible in at least one sliding window. Values are
 // the names of all lists (in no particular order) in which the given room ID is
diff --git a/sync3/lists_test.go b/sync3/lists_test.go
index c3c7a98..6917c29 100644
--- a/sync3/lists_test.go
+++ b/sync3/lists_test.go
@@ -31,6 +31,10 @@ func addRooms(list *sync3.InternalRequestLists, n int) {
 	for i := 0; i < n; i++ {
 		next := roomCounter.Add(1)
 		messageTimestamp := uint64(timestamp.Add(time.Duration(next) * time.Minute).UnixMilli())
+		lastInterestedEventTimestamps := make(map[string]uint64)
+		for _, listKey := range list.ListKeys() {
+			lastInterestedEventTimestamps[listKey] = messageTimestamp
+		}
 		list.SetRoom(sync3.RoomConnMetadata{
 			RoomMetadata: internal.RoomMetadata{
 				RoomID:               fmt.Sprintf("!%d:benchmark", next),
@@ -41,7 +45,7 @@ func addRooms(list *sync3.InternalRequestLists, n int) {
 			UserRoomData: caches.UserRoomData{
 				IsDM: next%10 == 0,
 			},
-			LastInterestedEventTimestamps: make(map[string]uint64),
+			LastInterestedEventTimestamps: lastInterestedEventTimestamps,
 		})
 	}
 }