diff --git a/BitFaster.Caching/Lfu/ConcurrentLfu.cs b/BitFaster.Caching/Lfu/ConcurrentLfu.cs
index 83a11e8e..d3410891 100644
--- a/BitFaster.Caching/Lfu/ConcurrentLfu.cs
+++ b/BitFaster.Caching/Lfu/ConcurrentLfu.cs
@@ -615,9 +615,9 @@ private LfuNode<K, V> EvictFromWindow()
             return first;
         }
 
-        private struct EvictIterator
+        private ref struct EvictIterator
         {
-            private CmSketch<K, DetectAvx2> sketch;
+            private readonly CmSketch<K, DetectAvx2> sketch;
             public LfuNode<K, V> node;
             public int freq;