diff --git a/persistence_windows/src/persistence_windows.rs b/persistence_windows/src/persistence_windows.rs
index 5b66a593f8..957d034d06 100644
--- a/persistence_windows/src/persistence_windows.rs
+++ b/persistence_windows/src/persistence_windows.rs
@@ -499,6 +499,10 @@ impl Window {
 
     /// Add one window to another. Used to collapse closed windows into persisted.
     fn add_window(&mut self, other: Self) {
+        assert!(self.last_instant <= other.created_at);
+        assert!(self.last_instant <= other.last_instant);
+
+        self.last_instant = other.last_instant;
         self.row_count += other.row_count;
         if self.min_time > other.min_time {
             self.min_time = other.min_time;
@@ -1327,7 +1331,8 @@ mod tests {
 
     #[test]
     fn test_summaries() {
-        let mut w = make_windows(Duration::from_secs(100));
+        let late_arrival_period = Duration::from_secs(100);
+        let mut w = make_windows(late_arrival_period);
         let instant = w.created_at_instant;
 
         // Window 1
@@ -1407,6 +1412,34 @@ mod tests {
                     row_count: 8
                 },
             ]
-        )
+        );
+
+        // Rotate first and second windows into persistable
+        w.rotate(instant + late_arrival_period + DEFAULT_CLOSED_WINDOW_PERIOD * 2);
+
+        let summaries: Vec<_> = w.summaries().collect();
+
+        assert_eq!(summaries.len(), 2);
+        assert_eq!(
+            summaries,
+            vec![
+                WriteSummary {
+                    time_of_first_write: w.created_at_time + chrono::Duration::milliseconds(1),
+                    time_of_last_write: w.created_at_time
+                        + closed_duration
+                        + chrono::Duration::milliseconds(1),
+                    min_timestamp: Utc.timestamp_nanos(1),
+                    max_timestamp: Utc.timestamp_nanos(340),
+                    row_count: 24
+                },
+                WriteSummary {
+                    time_of_first_write: w.created_at_time + closed_duration * 3,
+                    time_of_last_write: w.created_at_time + closed_duration * 3,
+                    min_timestamp: Utc.timestamp_nanos(3),
+                    max_timestamp: Utc.timestamp_nanos(4),
+                    row_count: 8
+                },
+            ]
+        );
     }
 }