RequestFactory Could not parse payload: payload[0] = N

After deploying my app on appengine, I get the following exception:

com.google.web.bindery.requestfactory.server.RequestFactoryServlet doPost: Unexpected error  java.lang.RuntimeException: Could not parse payload: payload[0] = N      at com.google.web.bindery.autobean.vm.impl.JsonSplittable.create(JsonSplittable.java:70)      at com.google.web.bindery.autobean.shared.impl.StringQuoter.create(StringQuoter.java:46)      at com.google.web.bindery.autobean.shared.ValueCodex$Type$7.encode(ValueCodex.java:122)      at com.google.web.bindery.autobean.shared.ValueCodex.encode(ValueCodex.java:315)      at com.google.web.bindery.autobean.shared.impl.AutoBeanCodexImpl$ValueCoder.extractSplittable(AutoBeanCodexImpl.java:500)      at com.google.web.bindery.autobean.shared.impl.AbstractAutoBean.setProperty(AbstractAutoBean.java:277)      at com.google.web.bindery.autobean.vm.impl.ProxyAutoBean.setProperty(ProxyAutoBean.java:253)      at com.google.web.bindery.autobean.vm.impl.BeanPropertyContext.set(BeanPropertyContext.java:44)      at com.google.web.bindery.requestfactory.server.Resolver$PropertyResolver.visitValueProperty(Resolver.java:154)      at com.google.web.bindery.autobean.vm.impl.ProxyAutoBean.traverseProperties(ProxyAutoBean.java:289)      at com.google.web.bindery.autobean.shared.impl.AbstractAutoBean.traverse(AbstractAutoBean.java:166)      at com.google.web.bindery.autobean.shared.impl.AbstractAutoBean.accept(AbstractAutoBean.java:101)      at com.google.web.bindery.requestfactory.server.Resolver.resolveClientValue(Resolver.java:395)      at com.google.web.bindery.requestfactory.server.SimpleRequestProcessor.processInvocationMessages(SimpleRequestProcessor.java:483)      at com.google.web.bindery.requestfactory.server.SimpleRequestProcessor.process(SimpleRequestProcessor.java:225)      at com.google.web.bindery.requestfactory.server.SimpleRequestProcessor.process(SimpleRequestProcessor.java:127)      at com.google.web.bindery.requestfactory.server.RequestFactoryServlet.doPost(RequestFactoryServlet.java:133)      at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)

Any idea of what the exception means? Thanks

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