diff --git a/web/src/message_list_data.ts b/web/src/message_list_data.ts
index 8d1324eef5..ba6a12a950 100644
--- a/web/src/message_list_data.ts
+++ b/web/src/message_list_data.ts
@@ -84,11 +84,11 @@ export class MessageListData {
         return this._items.length === 0;
     }
 
-    first(): Message {
+    first(): Message | undefined {
         return this._items[0];
     }
 
-    first_including_muted(): Message {
+    first_including_muted(): Message | undefined {
         return this._all_items[0];
     }
 
@@ -105,7 +105,7 @@ export class MessageListData {
             return false;
         }
 
-        return this.first().id <= msg_id && msg_id <= this.last()!.id;
+        return this.first()!.id <= msg_id && msg_id <= this.last()!.id;
     }
 
     ids_greater_or_equal_than(my_id: number): number[] {
@@ -335,7 +335,7 @@ export class MessageListData {
             const last = this.last_including_muted();
             if (last === undefined || msg.id > last.id) {
                 bottom_messages.push(msg);
-            } else if (msg.id < this.first_including_muted().id) {
+            } else if (msg.id < this.first_including_muted()!.id) {
                 top_messages.push(msg);
             } else {
                 interior_messages.push(msg);
diff --git a/web/src/stream_topic_history.ts b/web/src/stream_topic_history.ts
index a148b90d30..c0a3df5e97 100644
--- a/web/src/stream_topic_history.ts
+++ b/web/src/stream_topic_history.ts
@@ -42,7 +42,7 @@ export function all_topics_in_cache(sub: StreamSubscription): boolean {
     // we might be missing the oldest topics in this stream in our
     // cache.
     const first_cached_message = all_messages_data.first();
-    return first_cached_message.id <= sub.first_message_id;
+    return first_cached_message!.id <= sub.first_message_id;
 }
 
 export function is_complete_for_stream_id(stream_id: number): boolean {