WebApi查询最大凭证号报错

通过WebApi方式查询最大凭证号json如下
{"FormId":"GL_VOUCHER","TopRowCount":0,"Limit":20,"StartRow":0,"FilterString":"","OrderString":"","FieldKeys":"max(FVOUCHERGROUPNO)"}

使用接口是Kingdee.BOS.WebApi.ServicesStub.DynamicFormService.ExecuteBillQuery.common.kdsvc

报错信息如下：
response_error:{"InnerExWrapper":{"IsEmpty":false,"Data":{},"HelpLink":null,"Message":"Exception has been thrown by the target of an invocation.","Source":null,"StackTrace":" at System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeType typeOwner)\r\n at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)\r\n at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.ServiceExecutor.Execute(KDServiceContext context, ServiceType svcType, String[] paraValues, SerializerProxy serializeProxy, ServiceType beginMethod)","TargetSite":null,"ExceptionType":"System.Reflection.TargetInvocationException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089","InnerException":{"IsEmpty":false,"Data":{},"HelpLink":null,"Message":"Index was out of range. Must be non-negative and less than the size of the collection.\r\nParameter name: index","Source":null,"StackTrace":"\r\nServer stack trace: \r\n at Kingdee.BOS.App.Core.Query.QueryBuilder.BuildSQLWhere()\r\n at Kingdee.BOS.App.Core.Query.QueryBuilder.BuilderQuery()\r\n at Kingdee.BOS.App.Core.Query.QueryService.GetDynamicObjectCollection(Context ctx, QueryBuilderParemeter para, List`1 paramList)\r\n\r\nException rethrown at [0]: \r\n at Kingdee.BOS.WebApi.FormService.QueryBillService.ExecuteBillQuery(String data)\r\n at Kingdee.BOS.WebApi.ServicesStub.DynamicFormService.ExecuteBillQuery(String data)","TargetSite":null,"ExceptionType":"System.ArgumentOutOfRangeException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089","InnerException":null}},"ClassName":"Kingdee.BOS.ServiceFacade.ServiceException","Message":"Index was out of range. Must be non-negative and less than the size of the collection.\r\nParameter name: index","Data":null,"InnerException":null,"HelpURL":null,"StackTraceString":" at Kingdee.BOS.ServiceFacade.KDServiceFx.ServiceExecutor.Execute(KDServiceContext context, ServiceType svcType, String[] paraValues, SerializerProxy serializeProxy, ServiceType beginMethod)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.ExecuteServiceModule.OnProcess(KDServiceContext context)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.ModulePipeline.ExcuteRequest(KDServiceContext context)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.RequestExcuteRuntime.BeginRquest(RequestExtractor requestExtractor, WebContext webCtx)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.RequestExcuteRuntime.StartRequest(RequestExtractor requestExtractor, WebContext context)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.KDSVCHandler.ExecuteRequest(WebContext ctx, RequestExtractor requestExtractor)\r\n at Kingdee.BOS.ServiceFacade.KDServiceFx.KDSVCHandler.ProcessRequestInternal(WebContext webCtx, RequestExtractor requestExtractor)","RemoteStackTraceString":null,"RemoteStackIndex":0,"ExceptionMethod":"8\nExecute\nKingdee.BOS.ServiceFacade.KDServiceFx, Version=6.0.413.1, Culture=neutral, PublicKeyToken=null\nKingdee.BOS.ServiceFacade.KDServiceFx.ServiceExecutor\nSystem.String Execute(Kingdee.BOS.ServiceFacade.KDServiceFx.KDServiceContext, Kingdee.BOS.ServiceFacade.KDServiceFx.ServiceType, System.String[], Kingdee.BOS.ServiceFacade.SerializerProxy, Kingdee.BOS.ServiceFacade.KDServiceFx.ServiceType)","HResult":-2147467259,"Source":"Kingdee.BOS.ServiceFacade.KDServiceFx","WatsonBuckets":null,"_httpCode":500}

请问查询最大凭证号json串如何写？