diff --git a/src/directLine.ts b/src/directLine.ts
index 443e0416..c7c97e3f 100644
--- a/src/directLine.ts
+++ b/src/directLine.ts
@@ -315,6 +315,7 @@ export interface IActivity {
     eTag?: string,
     from: User,
     id?: string,
+    replyToId?: string,
     timestamp?: string,
     deliveryMode?: DeliveryMode
 }