diff --git a/lib/reducers/thread-activity-reducer.test.js b/lib/reducers/thread-activity-reducer.test.js
--- a/lib/reducers/thread-activity-reducer.test.js
+++ b/lib/reducers/thread-activity-reducer.test.js
@@ -39,6 +39,24 @@
     expect(result).toEqual(expectedState);
   });
 
+  test('should create new thread activity entry with only lastNavigatedTo field', () => {
+    const initialState = {};
+    const action = {
+      type: updateThreadLastNavigatedActionType,
+      payload: {
+        threadID: 'thread1',
+        time: 1639522317443,
+      },
+    };
+    const expectedState = {
+      thread1: {
+        lastNavigatedTo: 1639522317443,
+      },
+    };
+    const result = reduceThreadActivity(initialState, action);
+    expect(result).toEqual(expectedState);
+  });
+
   test('returns the initial state if the action type is not recognized', () => {
     const initialState = {
       thread1: {
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,