WebService 在传回 ArrayList 时说： XML 文档时出错

System.InvalidOperationException: 生成 XML 文档时出错。 ---> System.InvalidOperationException: CProxyTypeSpace_EntityRoleEntity_NHibernate_ProxyINHibernateProxy_System_Runtime_SerializationISerializable2 无法序列化，因为它没有无参数的构造函数。
   在 System.Xml.Serialization.TypeDesc.CheckSupported()
   在 System.Xml.Serialization.TypeScope.GetTypeDesc(Type type,MemberInfo source,Boolean directReference,Boolean throwOnError)
   在 System.Xml.Serialization.XmlSerializationWriter.CreateUnknownTypeException(Type type)
   在 System.Xml.Serialization.XmlSerializationWriter.WriteTypedPrimitive(String name,String ns,Object o,Boolean xsiType)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write1_Object(String n,Boolean isNullable,Boolean needType)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write6_ArrayOfAnyType(Object o)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.ArrayListSerializer.Serialize(Object objectToSerialize,XmlSerializationWriter writer)
   在 System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter,XmlSerializerNamespaces namespaces,String encodingStyle,String id)
   --- 内部异常堆栈跟踪的结尾 ---
   在 System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter,String id)
   在 System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter,Object o)
   在 System.Web.Services.Protocols.XmlReturnWriter.Write(HttpResponse response,Stream outputStream,Object returnValue)
   在 System.Web.Services.Protocols.HttpServerProtocol.WriteReturns(Object[] returnValues,Stream outputStream)
   在 System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
   在 System.Web.Services.Protocols.WebServiceHandler.Invoke()