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
@@ -302,9 +302,9 @@
 
 function pushInfoToCollapsableNotifInfo(pushInfo: PushInfo): {
   +usersToCollapseKeysToInfo: {
-    [string]: { [string]: CollapsableNotifInfo },
+    +[string]: { +[string]: CollapsableNotifInfo },
   },
-  +usersToCollapsableNotifInfo: { [string]: Array<CollapsableNotifInfo> },
+  +usersToCollapsableNotifInfo: { +[string]: Array<CollapsableNotifInfo> },
 } {
   const usersToCollapseKeysToInfo: {
     [string]: { [string]: CollapsableNotifInfo },
@@ -348,9 +348,9 @@
 
 function mergeUserToCollapsableInfo(
   usersToCollapseKeysToInfo: {
-    [string]: { [string]: CollapsableNotifInfo },
+    +[string]: { +[string]: CollapsableNotifInfo },
   },
-  usersToCollapsableNotifInfo: { [string]: Array<CollapsableNotifInfo> },
+  usersToCollapsableNotifInfo: { +[string]: Array<CollapsableNotifInfo> },
 ): { [string]: Array<CollapsableNotifInfo> } {
   const mergedUsersToCollapsableInfo = { ...usersToCollapsableNotifInfo };
   for (const userID in usersToCollapseKeysToInfo) {