diff --git a/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java b/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
index 5200388af8881..5c4cdadbfecf4 100644
--- a/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
+++ b/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
@@ -2546,7 +2546,7 @@ private static class MockMTAppender extends AbstractAppender {
         private final List<String> messages = Collections.synchronizedList(new ArrayList<>());
 
         List<String> messages() {
-            return messages;
+            return List.copyOf(messages);
         }
 
         MockMTAppender(final String name) throws IllegalAccessException {