diff --git a/datatrails-common-api/assets/v2/assets/listevents.proto b/datatrails-common-api/assets/v2/assets/listevents.proto
index ccc3e9a..c6847d9 100644
--- a/datatrails-common-api/assets/v2/assets/listevents.proto
+++ b/datatrails-common-api/assets/v2/assets/listevents.proto
@@ -141,7 +141,7 @@ message ListEventsRequest {
     ];
 
     // filter events by mmr index
-    int64 mmr_index = 24 [(validate.rules).int64.gte = 0];
+    optional int64 mmr_index = 24 [(validate.rules).int64.gte = 0];
 }
 
 message ListEventsResponse {