org.apache.cxf.interceptor.Fault: Cannot write part return. RPC/Literal parts cannot be null. (WS-I BP R2211)

    /**
     * Get a custom property of a site
     *
     * @param sessionid the id of a valid session
     * @param siteid    the id of the site you want to get the property from
     * @param propname  the name of the property you want
     * @return the property or blank if not found/property is blank
     * @throws RuntimeException
     */
    @WebMethod
    @Path("/getSiteProperty")
    @Produces("text/plain")
    @GET
    public String getSiteProperty(
            @WebParam(name = "sessionid", partName = "sessionid") @QueryParam("sessionid") String sessionid,
            @WebParam(name = "siteid", partName = "siteid") @QueryParam("siteid") String siteid,
            @WebParam(name = "propname", partName = "propname") @QueryParam("propname") String propname) {
       .
       .
       .

06-Dec-2018 11:40:32.517 WARN [http-nio-8980-exec-9] org.apache.cxf.phase.PhaseInterceptorChain.doLog Interceptor for {http://webservices.sakaiproject.org/}SakaiScriptService#{http://webservices.sakaiproject.org/}getSiteProperty has thrown exception, unwinding now
org.apache.cxf.interceptor.Fault: Cannot write part return. RPC/Literal parts cannot be null. (WS-I BP R2211)
	at org.apache.cxf.binding.soap.interceptor.RPCOutInterceptor.handleMessage(RPCOutInterceptor.java:103)
	at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:308)
	at org.apache.cxf.interceptor.OutgoingChainInterceptor.handleMessage(OutgoingChainInterceptor.java:83)
	at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:308)
	at org.apache.cxf.transport.ChainInitiationObserver.onMessage(ChainInitiationObserver.java:121)
	at org.apache.cxf.transport.http.AbstractHTTPDestination.invoke(AbstractHTTPDestination.java:267)
	at org.apache.cxf.transport.servlet.ServletController.invokeDestination(ServletController.java:234)
	at org.apache.cxf.transport.servlet.ServletController.invoke(ServletController.java:208)
	at org.apache.cxf.transport.servlet.ServletController.invoke(ServletController.java:160)
	at org.apache.cxf.transport.servlet.CXFNonSpringServlet.invoke(CXFNonSpringServlet.java:189)
	at org.apache.cxf.transport.servlet.AbstractHTTPServlet.handleRequest(AbstractHTTPServlet.java:303)
	at org.apache.cxf.transport.servlet.AbstractHTTPServlet.doPost(AbstractHTTPServlet.java:222)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
	at org.apache.cxf.transport.servlet.AbstractHTTPServlet.service(AbstractHTTPServlet.java:278)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.sakaiproject.util.RequestFilter.doFilter(RequestFilter.java:467)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:748)