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
@@ -1,6 +1,7 @@
 // @flow
 
 import { reduceThreadActivity } from './thread-activity-reducer.js';
+import { messageStorePruneActionType } from '../actions/message-actions.js';
 import { deleteKeyserverAccountActionTypes } from '../actions/user-actions.js';
 import { updateThreadLastNavigatedActionType } from '../types/thread-activity-types.js';
 
@@ -39,6 +40,41 @@
     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('should create new thread activity entry with only lastPruned field', () => {
+    const initialState = {};
+    const action = {
+      type: messageStorePruneActionType,
+      payload: {
+        threadIDs: ['thread1'],
+      },
+    };
+    const expectedState = {
+      thread1: {
+        lastPruned: expect.any(Number),
+      },
+    };
+    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
@@ -3,10 +3,16 @@
 export const updateThreadLastNavigatedActionType =
   'UPDATE_THREAD_LAST_NAVIGATED';
 
-export type ThreadActivityStoreEntry = {
-  +lastNavigatedTo: number, // millisecond timestamp
-  +lastPruned: number, // millisecond timestamp
-};
+export type ThreadActivityStoreEntry =
+  | {
+      +lastNavigatedTo: number, // millisecond timestamp
+      +lastPruned?: number, // millisecond timestamp
+    }
+  | {
+      +lastNavigatedTo?: number,
+      +lastPruned: number,
+    };
+
 export type ThreadActivityStore = {
   +[threadID: string]: ThreadActivityStoreEntry,
 };