diff --git a/src/schema/vocab.rnc b/src/schema/vocab.rnc
index e16c6b40a997ceb7d24bd7cfba8faf01c6f70872..4d41834816923cf581a60949662439105876d4eb 100644
--- a/src/schema/vocab.rnc
+++ b/src/schema/vocab.rnc
@@ -27,7 +27,7 @@ skos.Concept =
    element skos:Concept {
       rdfPrimarySubject
     & Label+
-    & skos.notation
+    & skos.notation?
     & skos.inScheme
     & skos.match
     & empty