diff --git a/server/src/main/java/org/opensearch/transport/protobuf/SearchHitsProtobuf.java b/server/src/main/java/org/opensearch/transport/protobuf/SearchHitsProtobuf.java
index b46181f3e55b4..76518a345a6c7 100644
--- a/server/src/main/java/org/opensearch/transport/protobuf/SearchHitsProtobuf.java
+++ b/server/src/main/java/org/opensearch/transport/protobuf/SearchHitsProtobuf.java
@@ -55,12 +55,16 @@ public void fromProtobufStream(StreamInput in) throws IOException {
     }
 
     SearchHitsProto toProto() {
-        SearchHitsProto.Builder builder = SearchHitsProto.newBuilder().setMaxScore(maxScore).setCollapseField(collapseField);
+        SearchHitsProto.Builder builder = SearchHitsProto.newBuilder().setMaxScore(maxScore);
 
         for (SearchHit hit : hits) {
             builder.addHits(new SearchHitProtobuf(hit).toProto());
         }
 
+        if (collapseField != null) {
+            builder.setCollapseField(collapseField);
+        }
+
         if (totalHits != null) {
             TotalHitsProto.Builder totHitsBuilder = TotalHitsProto.newBuilder()
                 .setRelation(totalHits.relation.ordinal())
diff --git a/server/src/main/proto/serde/SearchHitsProto.proto b/server/src/main/proto/serde/SearchHitsProto.proto
index 9a5055e685c2e..92fdef26732b1 100644
--- a/server/src/main/proto/serde/SearchHitsProto.proto
+++ b/server/src/main/proto/serde/SearchHitsProto.proto
@@ -23,7 +23,7 @@ Object[] collapseValues
  */
 message SearchHitsProto {
   float max_score = 1;
-  string collapse_field = 2;
+  optional string collapse_field = 2;
   bytes sort_fields = 3;
   bytes collapse_values = 4;
   optional TotalHitsProto total_hits = 5;