diff --git a/src/ruleset.rnc b/src/ruleset.rnc
index 31aa283e42ed8e8b4a1fffcca4400dcabc6d3271..08d32154b75e6184c8bffa69eabae084bb4921e9 100644
--- a/src/ruleset.rnc
+++ b/src/ruleset.rnc
@@ -155,9 +155,9 @@ MetsModsSerialization =
    element METS {
       MetsModsSerializationClass
     & MetsModsSerializationNamespace*
-    & MetsModsSerializationAnchorQuery
-    & MetsModsSerializationAnchorIdentifierMetadataType
-    & MetsModsSerializationValueRegExp
+    & MetsModsSerializationAnchorQuery?
+    & MetsModsSerializationAnchorIdentifierMetadataType?
+    & MetsModsSerializationValueRegExp?
     & MetsModsSerializationMetadataMapping+
     & MetsModsSerializationDocStructMapping+
    }