diff --git a/src/java/org/web3d/vrml/nodes/SceneGraphTraverser.java b/src/java/org/web3d/vrml/nodes/SceneGraphTraverser.java
index 94173be866932519fbbb857feeaa3c0589b2bfc5..f8140495dc71da45a5fb881c2338b4ec211ba373 100644
--- a/src/java/org/web3d/vrml/nodes/SceneGraphTraverser.java
+++ b/src/java/org/web3d/vrml/nodes/SceneGraphTraverser.java
@@ -39,7 +39,7 @@ public class SceneGraphTraverser {
     private boolean inUse;
 
     /** Temporary map during traversal for use references */
-    private HashSet<VRMLNodeType> usedNodes;
+    private final HashSet<VRMLNodeType> usedNodes;
 
     /** The detailObs for the scene graph */
     private SceneGraphTraversalDetailObserver detailObs;