diff --git a/lib/components/chat-mention-provider.react.js b/lib/components/chat-mention-provider.react.js
--- a/lib/components/chat-mention-provider.react.js
+++ b/lib/components/chat-mention-provider.react.js
@@ -25,7 +25,7 @@
 export type ChatMentionContextType = {
   +getChatMentionSearchIndex: (
     threadInfo: ThreadInfo,
-  ) => SentencePrefixSearchIndex,
+  ) => ?SentencePrefixSearchIndex,
   +communityThreadIDForGenesisThreads: { +[id: string]: string },
   +chatMentionCandidatesObj: ChatMentionCandidatesObj,
 };
diff --git a/lib/shared/mention-utils.js b/lib/shared/mention-utils.js
--- a/lib/shared/mention-utils.js
+++ b/lib/shared/mention-utils.js
@@ -133,7 +133,7 @@
 }
 
 function useMentionTypeaheadChatSuggestions(
-  chatSearchIndex: SentencePrefixSearchIndex,
+  chatSearchIndex: ?SentencePrefixSearchIndex,
   chatMentionCandidates: ChatMentionCandidates,
   typeaheadMatchedStrings: ?TypeaheadMatchedStrings,
 ): $ReadOnlyArray<MentionTypeaheadChatSuggestionItem> {
@@ -142,7 +142,11 @@
   return React.useMemo(() => {
     const result = [];
 
-    if (chatNamePrefix === undefined || chatNamePrefix === null) {
+    if (
+      chatNamePrefix === undefined ||
+      chatNamePrefix === null ||
+      !chatSearchIndex
+    ) {
       return result;
     }