diff --git a/model/content/UsageControlObject.ttl b/model/content/UsageControlObject.ttl
index 68d2314c..90ea4881 100644
--- a/model/content/UsageControlObject.ttl
+++ b/model/content/UsageControlObject.ttl
@@ -18,7 +18,7 @@ ids:UsageControlObject a owl:Class;
            ];
            idsm:validation [
                idsm:forProperty ids:contract;
-               idsm:constraint idsm:NotNull;
+               idsm:constraint idsm:NotEmpty;
                idsm:relationType idsm:OneToMany;
            ] ;
            idsm:validation [