diff --git a/lucene/src/java/org/apache/lucene/util/MapBackedSet.java b/lucene/src/java/org/apache/lucene/util/MapBackedSet.java
index 7b0c42c8ae3..9db05ec86ba 100644
--- a/lucene/src/java/org/apache/lucene/util/MapBackedSet.java
+++ b/lucene/src/java/org/apache/lucene/util/MapBackedSet.java
@@ -28,7 +28,7 @@ import java.util.Map;
  * 
  * @lucene.internal
  */
-public class MapBackedSet<E> extends AbstractSet<E> implements Serializable {
+public final class MapBackedSet<E> extends AbstractSet<E> implements Serializable {
 
   private static final long serialVersionUID = -6761513279741915432L;