In JaxRpcDIITestCase.testEchoSimpleUserType

the SOAPContentElement hits

SerializerSupport ser = (SerializerSupport)serializerFactory.getSerializer();
xmlFragment = ser.serialize(xmlName, xmlType, objectValue, serContext);

if (xmlFragment == null || xmlFragment.startsWith("<") == false)
throw new IllegalStateException("Cannot serialize objectValue: " + objectValue);

The marshalling layer should never return null for a given objectValue, instead there should be a meaningful exception about the cause of the problem