Web Services 服务端报 Non nillable element 'accountBalanceSet' is null
一个web services接口报以下错误。谁能给指点一下。

3:42:43,982 ERROR BillingSoapBindingImpl:747 - End accountBalanceQueryService
03:42:43,987 ERROR BillingSoapBindingImpl:748 - query failed, the reason is ：数据库查询错误
03:42:43,987 ERROR BillingSoapBindingImpl:965 - end accountBalanceQueryService
03:42:44,052 ERROR BeanSerializer:274 - Exception:
java.io.IOException: Non nillable element 'accountBalanceSet' is null.
at org.apache.axis.encoding.ser.BeanSerializer.serialize(BeanSerializer.java:215)
at org.apache.axis.encoding.SerializationContext.serializeActual(SerializationContext.java:1504)
at org.apache.axis.encoding.SerializationContext.serialize(SerializationContext.java:980)
at org.apache.axis.encoding.SerializationContext.outputMultiRefs(SerializationContext.java:1055)
at org.apache.axis.message.SOAPBody.outputImpl(SOAPBody.java:145)
at org.apache.axis.message.SOAPEnvelope.outputImpl(SOAPEnvelope.java:478)
at org.apache.axis.message.MessageElement.output(MessageElement.java:1208)
at org.apache.axis.SOAPPart.writeTo(SOAPPart.java:315)
at org.apache.axis.SOAPPart.writeTo(SOAPPart.java:269)
at org.apache.axis.SOAPPart.saveChanges(SOAPPart.java:530)
at org.apache.axis.attachments.AttachmentsImpl.getAttachmentCount(AttachmentsImpl.java:554)
at org.apache.axis.Message.getContentType(Message.java:486)
at org.apache.axis.transport.http.AxisServlet.doPost(AxisServlet.java:775)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:638)
at org.apache.axis.transport.http.AxisServletBase.service(AxisServletBase.java:327)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:720)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:199)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:145)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:210)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:955)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:139)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:955)
at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2460)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:133)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:119)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:116)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:955)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:127)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:955)
at org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:157)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:873)