diff --git a/src/ruleset.rnc b/src/ruleset.rnc
index 815e19f2ece3fc5eda0930c56868d0c6b823c385..3f2a9d0b6f92f02d770b2b67c6b52f64aa47cf0a 100644
--- a/src/ruleset.rnc
+++ b/src/ruleset.rnc
@@ -153,12 +153,12 @@ PicaPlusImportMappingElements =
 
 MetsModsSerialization =
    element METS {
-      MetsModsSerializationClass
+      MetsModsSerializationClass?
     & MetsModsSerializationNamespace*
     & MetsModsSerializationAnchorQuery?
     & MetsModsSerializationAnchorIdentifierMetadataType?
     & MetsModsSerializationValueRegExp?
-    & MetsModsSerializationMetadataMapping+
+    & MetsModsSerializationMetadataMapping*
     & MetsModsSerializationDocStructMapping+
    }