diff --git a/pmml-model/src/main/java/org/dmg/pmml/Version.java b/pmml-model/src/main/java/org/dmg/pmml/Version.java
index a4f6b3a6..cc4b39dd 100644
--- a/pmml-model/src/main/java/org/dmg/pmml/Version.java
+++ b/pmml-model/src/main/java/org/dmg/pmml/Version.java
@@ -4,25 +4,52 @@
 package org.dmg.pmml;
 
 public enum Version {
-	PMML_3_0("http://www.dmg.org/PMML-3_0"),
+	PMML_3_0("http://www.dmg.org/PMML-3_0"){
+
+		@Override
+		public Version previous(){
+			return null;
+		}
+	},
 	PMML_3_1("http://www.dmg.org/PMML-3_1"),
 	PMML_3_2("http://www.dmg.org/PMML-3_2"),
 	PMML_4_0("http://www.dmg.org/PMML-4_0"),
 	PMML_4_1("http://www.dmg.org/PMML-4_1"),
 	PMML_4_2("http://www.dmg.org/PMML-4_2"),
 	PMML_4_3("http://www.dmg.org/PMML-4_3"),
-	PMML_4_4("http://www.dmg.org/PMML-4_4"),
+	PMML_4_4("http://www.dmg.org/PMML-4_4"){
+
+		@Override
+		public Version next(){
+			return null;
+		}
+	},
 
 	/**
 	 * Extended PMML
 	 */
 	XPMML("http://xpmml.org/XPMML"){
 
+		@Override
+		public boolean isStandard(){
+			return false;
+		}
+
 		@Override
 		public String getVersion(){
 			throw new UnsupportedOperationException();
 		}
-	}
+
+		@Override
+		public Version previous(){
+			return null;
+		}
+
+		@Override
+		public Version next(){
+			return null;
+		}
+	},
 	;
 
 	private String namespaceURI = null;
@@ -33,9 +60,7 @@ private Version(String namespaceURI){
 	}
 
 	public boolean isStandard(){
-		String namespaceURI = getNamespaceURI();
-
-		return namespaceURI.matches(Version.REGEX_PMML_XMLNS);
+		return true;
 	}
 
 	public String getNamespaceURI(){
@@ -54,6 +79,18 @@ public String getVersion(){
 		return version.replace('_', '.');
 	}
 
+	public Version previous(){
+		Version[] versions = Version.values();
+
+		return versions[ordinal() - 1];
+	}
+
+	public Version next(){
+		Version[] versions = Version.values();
+
+		return versions[ordinal() + 1];
+	}
+
 	static
 	public Version getMinimum(){
 		Version[] versions = Version.values();
diff --git a/pmml-model/src/main/java/org/jpmml/model/visitors/VersionInspector.java b/pmml-model/src/main/java/org/jpmml/model/visitors/VersionInspector.java
index f8e152b3..8626b7a3 100644
--- a/pmml-model/src/main/java/org/jpmml/model/visitors/VersionInspector.java
+++ b/pmml-model/src/main/java/org/jpmml/model/visitors/VersionInspector.java
@@ -100,7 +100,7 @@ private void inspect(PMMLObject object, Field field, Object value){
 
 				Required required = field.getAnnotation(Required.class);
 				if(required != null){
-					updateMaximum(object, field, previous(required.value()));
+					updateMaximum(object, field, (required.value()).previous());
 				}
 
 				return;
@@ -133,11 +133,4 @@ private boolean isNull(Object value){
 
 		return (value == null);
 	}
-
-	static
-	private Version previous(Version version){
-		Version[] values = Version.values();
-
-		return values[version.ordinal() - 1];
-	}
 }
\ No newline at end of file