diff --git a/lib/types/thread-activity-types.js b/lib/types/thread-activity-types.js
--- a/lib/types/thread-activity-types.js
+++ b/lib/types/thread-activity-types.js
@@ -5,7 +5,7 @@
 
 export type ThreadActivityStoreEntry = {
   +lastNavigatedTo: number, // millisecond timestamp
-  +lastPruned: number, // millisecond timestamp
+  +lastPruned?: number, // millisecond timestamp
 };
 export type ThreadActivityStore = {
   +[threadID: string]: ThreadActivityStoreEntry,