diff --git a/src/main/java/com/github/davidmoten/rtree/Backpressure.java b/src/main/java/com/github/davidmoten/rtree/Backpressure.java
index c3773db1..1379e16d 100644
--- a/src/main/java/com/github/davidmoten/rtree/Backpressure.java
+++ b/src/main/java/com/github/davidmoten/rtree/Backpressure.java
@@ -10,8 +10,9 @@
 class Backpressure {
 
     static <T> ImmutableStack<NodePosition<T>> search(Node<T> node,
-            Func1<? super Geometry, Boolean> condition, Subscriber<? super Entry<T>> subscriber,
-            ImmutableStack<NodePosition<T>> stack, long request) {
+            final Func1<? super Geometry, Boolean> condition,
+            final Subscriber<? super Entry<T>> subscriber, ImmutableStack<NodePosition<T>> stack,
+            long request) {
         while (true) {
             Preconditions.checkArgument(!stack.isEmpty());
             NodePosition<T> np = stack.peek();