Getting a 500 NullPointerException in Websphere - Apache Wink Rest services

I'm calling this in websphere 8.5

@GET 
@Path("/test")
@Produces("application/json")
public dtObject tester()
{
    dtObject r = new dtObject();
    r.successfulOperation();
    return r;
}

the dtObject class

@JsonSerialize(include=JsonSerialize.Inclusion.NON_NULL)
@XmlRootElement
public class dtObject {
private String success;
private String message;

public dtObject() {

}

public void successfulOperation ()
{
    this.message = "Ok";
    this.success = "true";
}

and the server responses:

error 500 java.lang.nullpointerexception

in logs:

RequestProces E org.apache.wink.server.internal.RequestProcessor     handleRequest An unhandled exception occurred which will be propagated to the   container.
java.lang.NullPointerException
at org.apache.wink.common.internal.http.Accept.valueOf(Accept.java:139)
at org.apache.wink.server.internal.contexts.HttpHeadersImpl.getAcceptHeader(HttpHeadersImpl.java:151)
at org.apache.wink.server.internal.contexts.HttpHeadersImpl.getAcceptableMediaTypes(HttpHeadersImpl.java:105)
at org.apache.wink.server.internal.registry.ResourceRegistry.filterByProduces(ResourceRegistry.java:573)
at org.apache.wink.server.internal.registry.ResourceRegistry.filterDispatchMethods(ResourceRegistry.java:497)
at org.apache.wink.server.internal.registry.ResourceRegistry.findSubResourceMethod(ResourceRegistry.java:391)
at org.apache.wink.server.internal.handlers.FindResourceMethodHandler.handleSubResourceMethod(FindResourceMethodHandler.java:168)
at org.apache.wink.server.internal.handlers.FindResourceMethodHandler.handleRequest(FindResourceMethodHandler.java:110)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.internal.handlers.FindRootResourceHandler.handleRequest(FindRootResourceHandler.java:95)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.internal.handlers.HeadMethodHandler.handleRequest(HeadMethodHandler.java:53)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.internal.handlers.OptionsMethodWADLHandler.handleRequest(OptionsMethodWADLHandler.java:51)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.internal.handlers.SearchResultHandler.handleRequest(SearchResultHandler.java:33)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.internal.log.ResourceInvocation.handleRequest(ResourceInvocation.java:92)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.internal.log.Requests.handleRequest(Requests.java:76)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:26)
at org.apache.wink.server.handlers.RequestHandlersChain.handle(RequestHandlersChain.java:22)
at org.apache.wink.server.handlers.AbstractHandlersChain.doChain(AbstractHandlersChain.java:75)
at org.apache.wink.server.handlers.AbstractHandlersChain.run(AbstractHandlersChain.java:60)
at org.apache.wink.server.internal.RequestProcessor.handleRequestWithoutFaultBarrier(RequestProcessor.java:207)
at org.apache.wink.server.internal.RequestProcessor.handleRequest(RequestProcessor.java:154)
at org.apache.wink.server.internal.servlet.RestServlet.service(RestServlet.java:124)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:668)
at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1230)
at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:779)
at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:478)
at com.ibm.ws.webcontainer.servlet.ServletWrapperImpl.handleRequest(ServletWrapperImpl.java:178)
at com.ibm.ws.webcontainer.filter.WebAppFilterManager.invokeFilters(WebAppFilterManager.java:1071)
at com.ibm.ws.webcontainer.webapp.WebApp.handleRequest(WebApp.java:3837)
at com.ibm.ws.webcontainer.webapp.WebGroup.handleRequest(WebGroup.java:304)
at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:981)
at com.ibm.ws.webcontainer.WSWebContainer.handleRequest(WSWebContainer.java:1662)
at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:200)
at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:459)
at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewRequest(HttpInboundLink.java:526)
at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.processRequest(HttpInboundLink.java:312)
at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.ready(HttpInboundLink.java:283)
at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.sendToDiscriminators(NewConnectionInitialReadCallback.java:214)
at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.complete(NewConnectionInitialReadCallback.java:113)
at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:175)
at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217)
at com.ibm.io.async.AsyncChannelFuture.fireCompletionActions(AsyncChannelFuture.java:161)
at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:138)
at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:204)
at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:775)
at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:905)
at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1864)

only one server have this behavior i've tested in another three servers and my services are running well there.

I also have restarted websphere as is suggested here http://ift.tt/1BBcweZ

So i guess its most a server side problem but i don't know exactly why is this happening