diff --git a/src/api/initialDataTypes.js b/src/api/initialDataTypes.js
index 3f0c3963b50..68538cc6827 100644
--- a/src/api/initialDataTypes.js
+++ b/src/api/initialDataTypes.js
@@ -511,12 +511,9 @@ export type InitialDataUserStatus = $ReadOnly<{|
    */
   // TODO(server-1.9.1): Make required.
   user_status?: $ReadOnly<{|
-    // TODO(flow): The key here is really UserId, not just any number; but
-    //   this Flow bug:
-    //     https://github.com/facebook/flow/issues/5407
-    //   means that doesn't work right, and the best workaround is to
-    //   leave it as `number`.
-    [userId: number]: $ReadOnly<{|
+    // Keys are UserId encoded as strings (just because JS objects are
+    // string-keyed).
+    [userId: string]: $ReadOnly<{|
       // TODO: add status emoji properties
       // TODO: Comment on what these mean (if doc not fixed):
       //   https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/Emoji.20statuses.20in.20zulip.2Eyaml/near/1322329
diff --git a/src/user-status/userStatusReducer.js b/src/user-status/userStatusReducer.js
index 4a3a9dc9bc1..9048de185e4 100644
--- a/src/user-status/userStatusReducer.js
+++ b/src/user-status/userStatusReducer.js
@@ -1,4 +1,8 @@
 /* @flow strict-local */
+
+import { objectFromEntries } from '../jsBackport';
+import { makeUserId } from '../api/idTypes';
+import objectEntries from '../utils/objectEntries';
 import type { UserStatusState, PerAccountApplicableAction } from '../types';
 import {
   LOGOUT,
@@ -22,7 +26,18 @@ export default (
       return initialState;
 
     case REGISTER_COMPLETE:
-      return action.data.user_status || initialState;
+      return objectFromEntries(
+        objectEntries(action.data.user_status ?? {}).map(([id, status]) => [
+          // Converting from string keys to numeric ones here doesn't
+          // actually make a difference to how the state is represented
+          // at runtime. But it will when we start using
+          // Immutable.Map<UserId, UserStatus> soon, and it satisfies
+          // Flow.
+          // TODO: Use Immutable.Map<UserId, UserStatus>
+          makeUserId(Number.parseInt(id, 10)),
+          status,
+        ]),
+      );
 
     case EVENT_USER_STATUS_UPDATE: {
       const newUserStatus = { ...state[action.user_id] };