diff --git a/core/src/main/java/zingg/hash/Round.java b/core/src/main/java/zingg/hash/Round.java
index d6d1783ec..22739f8b9 100644
--- a/core/src/main/java/zingg/hash/Round.java
+++ b/core/src/main/java/zingg/hash/Round.java
@@ -13,7 +13,7 @@ public Round() {
 
 	 @Override
 	 public Long call(Double field) {
-		 return Math.round(field);
+		 return field == null ? null : Math.round(field);
 	 }
 
 	 @Override