diff --git a/keyserver/src/push/send.js b/keyserver/src/push/send.js
--- a/keyserver/src/push/send.js
+++ b/keyserver/src/push/send.js
@@ -777,7 +777,9 @@
 
 async function fetchNotifUserInfos(
   serverThreadInfos: { +[threadID: string]: ServerThreadInfo },
-  usersToCollapsableNotifInfo: { +[userID: string]: CollapsableNotifInfo[] },
+  usersToCollapsableNotifInfo: {
+    +[userID: string]: $ReadOnlyArray<CollapsableNotifInfo>,
+  },
 ) {
   const missingUserIDs = new Set<string>();
 
diff --git a/lib/push/send-utils.js b/lib/push/send-utils.js
--- a/lib/push/send-utils.js
+++ b/lib/push/send-utils.js
@@ -94,7 +94,7 @@
 };
 
 export type FetchCollapsableNotifsResult = {
-  [userID: string]: CollapsableNotifInfo[],
+  +[userID: string]: $ReadOnlyArray<CollapsableNotifInfo>,
 };
 
 function identityPlatformDetailsToPlatformDetails(
@@ -302,9 +302,11 @@
 
 function pushInfoToCollapsableNotifInfo(pushInfo: PushInfo): {
   +usersToCollapseKeysToInfo: {
-    [string]: { [string]: CollapsableNotifInfo },
+    +[string]: { +[string]: CollapsableNotifInfo },
+  },
+  +usersToCollapsableNotifInfo: {
+    +[string]: $ReadOnlyArray<CollapsableNotifInfo>,
   },
-  +usersToCollapsableNotifInfo: { [string]: Array<CollapsableNotifInfo> },
 } {
   const usersToCollapseKeysToInfo: {
     [string]: { [string]: CollapsableNotifInfo },
@@ -348,13 +350,23 @@
 
 function mergeUserToCollapsableInfo(
   usersToCollapseKeysToInfo: {
-    [string]: { [string]: CollapsableNotifInfo },
+    +[string]: { +[string]: CollapsableNotifInfo },
+  },
+  usersToCollapsableNotifInfo: {
+    +[string]: $ReadOnlyArray<CollapsableNotifInfo>,
   },
-  usersToCollapsableNotifInfo: { [string]: Array<CollapsableNotifInfo> },
-): { [string]: Array<CollapsableNotifInfo> } {
-  const mergedUsersToCollapsableInfo = { ...usersToCollapsableNotifInfo };
+): { +[string]: $ReadOnlyArray<CollapsableNotifInfo> } {
+  const mergedUsersToCollapsableInfo: {
+    [string]: Array<CollapsableNotifInfo>,
+  } = {};
+
   for (const userID in usersToCollapseKeysToInfo) {
     const collapseKeysToInfo = usersToCollapseKeysToInfo[userID];
+
+    mergedUsersToCollapsableInfo[userID] = [
+      ...usersToCollapsableNotifInfo[userID],
+    ];
+
     for (const collapseKey in collapseKeysToInfo) {
       const info = collapseKeysToInfo[collapseKey];
       mergedUsersToCollapsableInfo[userID].push({