diff --git a/src/test/java/net/openhft/chronicle/map/MemoryLeaksTest.java b/src/test/java/net/openhft/chronicle/map/MemoryLeaksTest.java
index d0174a274..801530a28 100644
--- a/src/test/java/net/openhft/chronicle/map/MemoryLeaksTest.java
+++ b/src/test/java/net/openhft/chronicle/map/MemoryLeaksTest.java
@@ -60,7 +60,7 @@ public class MemoryLeaksTest {
         this.persisted = persisted;
         this.closeWithinContext = closeWithinContext;
         builder = ChronicleMap
-                .of(IntValue.class, String.class)
+                .of(IntValue.class, String.class).constantKeySizeBySample(Values.newHeapInstance(IntValue.class))
                 .valueReaderAndDataAccess(new CountedStringReader(), new StringUtf8DataAccess());
         if (replicated)
             builder.replication((byte) 1);