diff --git a/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java b/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java
index c95c31c51..62b9731dc 100644
--- a/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java
+++ b/dhp-common/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/MergeUtils.java
@@ -270,8 +270,9 @@ public class MergeUtils {
 		}
 
 		HashMap<String, KeyValue> values = new HashMap<>();
-		left.forEach(kv -> values.put(kv.getKey(), kv));
-		right.forEach(kv -> values.putIfAbsent(kv.getKey(), kv));
+
+		Optional.ofNullable(left).ifPresent(l -> l.forEach(kv -> values.put(kv.getKey(), kv)));
+		Optional.ofNullable(right).ifPresent(r -> r.forEach(kv -> values.putIfAbsent(kv.getKey(), kv)));
 
 		return new ArrayList<>(values.values());
 	}