diff --git a/lib/reducers/entry-reducer.js b/lib/reducers/entry-reducer.js
--- a/lib/reducers/entry-reducer.js
+++ b/lib/reducers/entry-reducer.js
@@ -290,7 +290,7 @@
     invariant(localID, 'localID should be set in CREATE_LOCAL_ENTRY');
     const newEntryInfos = {
       ...entryInfos,
-      [localID]: entryInfo,
+      [(localID: string)]: entryInfo,
     };
     const dayString = dateString(
       entryInfo.year,
@@ -405,7 +405,7 @@
     invariant(id, 'either serverID or localID should be set');
     const newEntryInfos = {
       ...entryInfos,
-      [id]: {
+      [(id: string)]: {
         ...entryInfos[id],
         deleted: true,
       },
diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -898,7 +898,7 @@
       messageStore,
       [action.payload.messageInfo],
       {
-        [action.payload.messageInfo.threadID]:
+        [(action.payload.messageInfo.threadID: string)]:
           messageTruncationStatus.UNCHANGED,
       },
       newThreadInfos,
@@ -929,7 +929,9 @@
     return mergeNewMessages(
       messageStore,
       action.payload.newMessageInfos,
-      { [action.payload.threadID]: messageTruncationStatus.UNCHANGED },
+      {
+        [(action.payload.threadID: string)]: messageTruncationStatus.UNCHANGED,
+      },
       newThreadInfos,
       action.type,
     );
@@ -1002,20 +1004,23 @@
     let updatedThreads;
     let local = { ...messageStore.local };
     if (messageStore.messages[localID]) {
-      const messages = { ...messageStore.messages, [localID]: payload };
+      const messages = {
+        ...messageStore.messages,
+        [(localID: string)]: payload,
+      };
       local = _pickBy(
         (localInfo: LocalMessageInfo, key: string) => key !== localID,
       )(messageStore.local);
       const thread = messageStore.threads[threadID];
       updatedThreads = {
-        [threadID]: {
+        [(threadID: string)]: {
           messageIDs: sortMessageIDs(messages)(messageIDs),
           startReached: thread?.startReached ?? true,
         },
       };
     } else {
       updatedThreads = {
-        [threadID]: messageStore.threads[threadID]
+        [(threadID: string)]: messageStore.threads[threadID]
           ? {
               ...messageStore.threads[threadID],
               messageIDs: [localID, ...messageIDs],
@@ -1072,7 +1077,7 @@
         threads: messageStore.threads,
         local: {
           ...messageStore.local,
-          [localID]: { sendFailed: true },
+          [(localID: string)]: { sendFailed: true },
         },
         currentAsOf: messageStore.currentAsOf,
       },
@@ -1545,7 +1550,7 @@
       {
         type: 'replace_threads',
         payload: {
-          threads: { [threadID]: threadState },
+          threads: { [(threadID: string)]: threadState },
         },
       },
     ];
@@ -1559,7 +1564,7 @@
       ...messageStore,
       threads: {
         ...messageStore.threads,
-        [threadID]: threadState,
+        [(threadID: string)]: threadState,
       },
       messages: processedMessageStore.messages,
     };
@@ -1663,7 +1668,7 @@
         messageIDsToBeRemoved.push(id);
         threads = {
           ...threads,
-          [threadID]: {
+          [(threadID: string)]: {
             ...existingThread,
             messageIDs: existingThread.messageIDs.filter(
               curMessageID => curMessageID !== id,
@@ -1676,14 +1681,14 @@
       } else if (!existingMessageIDs.has(id)) {
         threads = {
           ...threads,
-          [threadID]: {
+          [(threadID: string)]: {
             ...existingThread,
             messageIDs: [...existingThread.messageIDs, id],
           },
         };
         threadsNeedMsgIDsResorting.add(threadID);
       } else if (!threads[threadID]) {
-        threads = { ...threads, [threadID]: existingThread };
+        threads = { ...threads, [(threadID: string)]: existingThread };
       }
     }