diff --git a/ably.d.ts b/ably.d.ts
index 1627a9b540..2b4e7f3ace 100644
--- a/ably.d.ts
+++ b/ably.d.ts
@@ -2793,7 +2793,7 @@ declare namespace Types {
     /**
      * The connection ID of the publisher of this message.
      */
-    connectionId: string;
+    connectionId?: string;
     /**
      * The message payload, if provided.
      */