How can I make a parameter type for which I cannot annotate nillable in my JAX-WS endpoint?

Issue

• How can I make a parameter type for which I cannot annotate nillable in my JAX-WS endpoint?
• I want to use java.util.Calendar as a parameter in one of my methods in my JAX-WS interface, and I want it to be nillable. When I use xsi:nil="true" in the SOAP request, I get an IllegalArgumentException from JAXB during unmarshalling of the SOAP request.

Caused by: java.lang.IllegalArgumentException:
        at org.apache.xerces.jaxp.datatype.XMLGregorianCalendarImpl$Parser.parseYear(Unknown Source)
        at org.apache.xerces.jaxp.datatype.XMLGregorianCalendarImpl$Parser.parse(Unknown Source)
        at org.apache.xerces.jaxp.datatype.XMLGregorianCalendarImpl.<init>(Unknown Source)
        at org.apache.xerces.jaxp.datatype.DatatypeFactoryImpl.newXMLGregorianCalendar(Unknown Source)
        at __redirected.__DatatypeFactory.newXMLGregorianCalendar(__DatatypeFactory.java:181) [jboss-modules.jar:1.1.2.GA-redhat-1]
        at com.sun.xml.bind.DatatypeConverterImpl._parseDateTime(DatatypeConverterImpl.java:369)
        at com.sun.xml.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$2.parse(RuntimeBuiltinLeafInfoImpl.java:270)
        at com.sun.xml.bind.v2.model.impl.RuntimeBuiltinLeafInfoImpl$2.parse(RuntimeBuiltinLeafInfoImpl.java:268)
        at com.sun.xml.bind.v2.runtime.reflect.TransducedAccessor$CompositeTransducedAccessorImpl.parse(TransducedAccessor.java:247)
        at com.sun.xml.bind.v2.runtime.unmarshaller.LeafPropertyLoader.text(LeafPropertyLoader.java:65)
        ... 35 more

How can I fix this?