Union Codec serialize throws Class cast when the union object is passed

I have a union type in my yang schema

leaf port-number {
type union

}

through which it creates a PortNumber object.

But the Union codec UnionCodecStringImpl.serialize accepts a String and not the union object PortNumber.

The serialize() should be able to look for the types within the UnionTypeDefinition and return back the non-null value of the union.

Following is the error i am getting:
osgi> 2014-05-05 18:25:52.634 PDT [http-bio-8080-exec-8] ERROR o.o.y.y.d.i.c.xml.XmlDocumentUtils - Provided node value PortNumber [_string=LOCAL] did not have type type (urn:ietf:params:xml:ns:yang:1)union (types=[, uint32, string] required by mapping. Using stream instead.
java.lang.ClassCastException: org.opendaylight.yang.gen.v1.urn.opendaylight.flow.types.port.rev130925.CommonPort$PortNumber cannot be cast to java.lang.String
at org.opendaylight.yangtools.yang.data.impl.codec.TypeDefinitionAwareCodec$UnionCodecStringImpl.serialize(TypeDefinitionAwareCodec.java:548) ~[bundlefile:na]
at org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils.writeValueByType(XmlDocumentUtils.java:257) [bundlefile:na]
at org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils.writeValueByType(XmlDocumentUtils.java:218) [bundlefile:na]
at org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils.createXmlRootElement(XmlDocumentUtils.java:168) [bundlefile:na]
at org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils.createXmlRootElement(XmlDocumentUtils.java:189) [bundlefile:na]
at org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils.createXmlRootElement(XmlDocumentUtils.java:189) [bundlefile:na]
at org.opendaylight.yangtools.yang.data.impl.codec.xml.XmlDocumentUtils.toDocument(XmlDocumentUtils.java:112) [bundlefile:na]