diff --git a/alerting/src/main/kotlin/org/opensearch/alerting/transport/TransportGetFindingsSearchAction.kt b/alerting/src/main/kotlin/org/opensearch/alerting/transport/TransportGetFindingsSearchAction.kt
index bf7a076e2..2720dc30b 100644
--- a/alerting/src/main/kotlin/org/opensearch/alerting/transport/TransportGetFindingsSearchAction.kt
+++ b/alerting/src/main/kotlin/org/opensearch/alerting/transport/TransportGetFindingsSearchAction.kt
@@ -203,7 +203,7 @@ class TransportGetFindingsSearchAction @Inject constructor(
         }
     }
 
-    private fun addDocument(findingDocument: FindingDocument, docs: MutableList<FindingDocument>) {
+    private fun addDocument(findingDocument: FindingDocument, docs: List<FindingDocument>) {
         docs.add(findingDocument)
     }
 }