I have a document style web service with a problem. I'm obviously doing something wrong but I can't figure out what is wrong. Whenever the service is invoked I get a org.jboss.ws.WSException stating "javaType cannot be null". Why does this happen?

The service implements this interface:

package decs2.pi;

public interface ProcessInitiator extends java.rmi.Remote
{
 public javax.xml.soap.SOAPElement invoke(javax.xml.soap.SOAPElement msg) throws java.rmi.RemoteException;
}

<definitions name='ProcessInitiatorService' targetNamespace='http://decs2/jaws' xmlns='http://schemas.xmlsoap.org/wsdl/' xmlns:decs2='http://decs2/jaws' xmlns:soap='http://schemas.xmlsoap.org/wsdl/soap/' xmlns:typeNS='http://decs2/jaws/types'>
 <types>
 <schema targetNamespace='http://decs2/jaws/types' xmlns='http://www.w3.org/2001/XMLSchema'>
 <element name='subtractProcessOp' type='typeNS:subtractProcessOpType'/>
 <complexType name='subtractProcessOpType'>
 <sequence>
 <element name='x' type='int'></element>
 <element name='y' type='int'></element>
 </sequence>
 </complexType>
 <element name='subtractProcessOpResponse' type='typeNS:subtractProcessOpResponseType'/>
 <complexType name='subtractProcessOpResponseType'>
 <sequence>
 <element name='result' type='int'></element>
 </sequence>
 </complexType>
 </schema>
 </types>
 <message name='INPUT'>
 <part element='typeNS:subtractProcessOp' name='parameters'/>
 </message>
 <message name='OUTPUT'>
 <part element='typeNS:subtractProcessOpResponse' name='result'/>
 </message>
 <portType name='ProcessInitiator'>
 <operation name='subtractProcessOp'>
 <input message='decs2:INPUT'/>
 <output message='decs2:OUTPUT'/>
 </operation>
 </portType>
 <binding name='ProcessInitiatorBinding' type='decs2:ProcessInitiator'>
 <soap:binding style='document' transport='http://schemas.xmlsoap.org/soap/http'/>
 <operation name='subtractProcessOp'>
 <soap:operation soapAction=''/>
 <input>
 <soap:body use='literal'/>
 </input>
 <output>
 <soap:body use='literal'/>
 </output>
 </operation>
 </binding>
 <service name='ProcessInitiatorService'>
 <port binding='decs2:ProcessInitiatorBinding' name='ProcessInitiatorPort'>
 <soap:address location='REPLACE_WITH_ACTUAL_URL'/>
 </port>
 </service>
</definitions>

<java-wsdl-mapping version='1.1' xmlns='http://java.sun.com/xml/ns/j2ee' xmlns:decs2='http://decs2/jaws'>
 <package-mapping>
 <package-type>decs2</package-type>
 <namespaceURI>http://decs2/jaws/types</namespaceURI>
 </package-mapping>

 <java-xml-type-mapping>
 <java-type>javax.xml.soap.SOAPElement</java-type>
 <root-type-qname xmlns:typeNS='http://decs2/jaws/types'>typeNS:subtractProcessOpType</root-type-qname>
 <qname-scope>complexType</qname-scope>
 </java-xml-type-mapping>

 <java-xml-type-mapping>
 <java-type>javax.xml.soap.SOAPElement</java-type>
 <root-type-qname xmlns:typeNS='http://decs2/jaws/types'>typeNS:subtractProcessOpResponseType</root-type-qname>
 <qname-scope>complexType</qname-scope>
 </java-xml-type-mapping>

 <service-interface-mapping>
 <service-interface>decs2.ProcessInitiatorService</service-interface>
 <wsdl-service-name>decs2:ProcessInitiatorService</wsdl-service-name>
 <port-mapping>
 <port-name>ProcessInitiatorPort</port-name>
 <java-port-name>ProcessInitiatorPort</java-port-name>
 </port-mapping>
 </service-interface-mapping>

 <service-endpoint-interface-mapping>
 <service-endpoint-interface>decs2.pi.ProcessInitiator</service-endpoint-interface>
 <wsdl-port-type>decs2:ProcessInitiator</wsdl-port-type>
 <wsdl-binding>decs2:ProcessInitiatorBinding</wsdl-binding>
 <service-endpoint-method-mapping>
 <java-method-name>invoke</java-method-name>
 <wsdl-operation>subtractProcessOp</wsdl-operation>
 <wrapped-element/>
 <method-param-parts-mapping>
 <param-position>0</param-position>
 <param-type>javax.xml.soap.SOAPElement</param-type>
 <wsdl-message-mapping>
 <wsdl-message>decs2:INPUT</wsdl-message>
 <wsdl-message-part-name>parameters</wsdl-message-part-name>
 <parameter-mode>IN</parameter-mode>
 </wsdl-message-mapping>
 </method-param-parts-mapping>
 <wsdl-return-value-mapping>
 <method-return-value>javax.xml.soap.SOAPElement</method-return-value>
 <wsdl-message>decs2:OUTPUT</wsdl-message>
 <wsdl-message-part-name>result</wsdl-message-part-name>
 </wsdl-return-value-mapping>
 </service-endpoint-method-mapping>
 </service-endpoint-interface-mapping>
</java-wsdl-mapping>