diff --git a/dhp-schemas/src/main/java/eu/dnetlib/dhp/schema/oaf/OafEntity.java b/dhp-schemas/src/main/java/eu/dnetlib/dhp/schema/oaf/OafEntity.java
index 09742748d..2823ee49d 100644
--- a/dhp-schemas/src/main/java/eu/dnetlib/dhp/schema/oaf/OafEntity.java
+++ b/dhp-schemas/src/main/java/eu/dnetlib/dhp/schema/oaf/OafEntity.java
@@ -106,6 +106,7 @@ public abstract class OafEntity extends Oaf implements Serializable {
 			.stream(lists)
 			.filter(Objects::nonNull)
 			.flatMap(List::stream)
+			.filter(Objects::nonNull)
 			.distinct()
 			.collect(Collectors.toList());
 	}