diff --git a/lib/shared/unshim-utils.js b/lib/shared/unshim-utils.js
--- a/lib/shared/unshim-utils.js
+++ b/lib/shared/unshim-utils.js
@@ -21,7 +21,7 @@
 
 function unshimFunc(
   messageInfo: RawMessageInfo,
-  unshimTypes: Set<MessageType>,
+  unshimTypes: $ReadOnlySet<MessageType>,
 ): RawMessageInfo {
   if (messageInfo.type !== messageTypes.UNSUPPORTED) {
     return messageInfo;
@@ -46,7 +46,7 @@
   return { ...messageStore, messages };
 }
 
-const localUnshimTypes = new Set([
+const localUnshimTypes: $ReadOnlySet<MessageType> = new Set([
   messageTypes.IMAGES,
   messageTypes.LEGACY_UPDATE_RELATIONSHIP,
   messageTypes.CREATE_SIDEBAR,