diff --git a/src/api/initialDataTypes.js b/src/api/initialDataTypes.js
index 944ef0f5a55..e8d5ee1afb0 100644
--- a/src/api/initialDataTypes.js
+++ b/src/api/initialDataTypes.js
@@ -504,12 +504,7 @@ 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<{|
+    [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..f1946aba755 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 +26,20 @@ export default (
       return initialState;
 
     case REGISTER_COMPLETE:
-      return action.data.user_status || initialState;
+      return action.data.user_status
+        ? 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,
+            ]),
+          )
+        : initialState;
 
     case EVENT_USER_STATUS_UPDATE: {
       const newUserStatus = { ...state[action.user_id] };