diff --git a/src/main/java/org/gcube/informationsystem/base/reference/Element.java b/src/main/java/org/gcube/informationsystem/base/reference/Element.java
index c30be5d..acb65fb 100644
--- a/src/main/java/org/gcube/informationsystem/base/reference/Element.java
+++ b/src/main/java/org/gcube/informationsystem/base/reference/Element.java
@@ -42,7 +42,7 @@ public interface Element extends Serializable {
 	@JsonIgnore
 	public String getTypeName();
 	
-	@JsonInclude(Include.NON_NULL)
+	@JsonInclude(Include.NON_EMPTY)
 	@JsonGetter(value = SUPERTYPES_PROPERTY)
 	public List<String> getSupertypes();