C # – How to return a generic entity in ToList () format [WebMethod]

[WebMethod]

public list QueryReturnList (sql)
DataTable objDTable = DB.ConsultViaDataTable (sql);
var lista1 = ConvertDataTableToList (objDTable);

                            retur list1 err here 

System.InvalidOperationException: An error occurred while generating the XML document. —> System.InvalidOperationException: The type ClassEntity.Customer was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.
on System.Xml.Serialization.XmlSerializationWriter.WriteTypedPrimitive (string name, string ns, object o, boolean type xsiType)
in Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write1_Object (String n, ns String, Object o, Boolean isNullable, Boolean needType)
on Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write8_ArrayOfAnyType (Object o)
on the Microsoft.Xml.Serialization.GeneratedAssembly.ListOfObjectSerializer.Serialize (Object objectToSerialize, Editor XmlSerializationWriter)
in System.Xml.Serialization.XmlSerializer.Serialize (xmlWriter, Object o, XmlSerializerNamespaces Namespaces, String encodingStyle, String id)
— Trace end of stack of internal exceptions —
in System.Xml.Serialization.XmlSerializer.Serialize (xmlWriter, Object o, XmlSerializerNamespaces Namespaces, String encodingStyle, String id)
in System.Xml.Serialization.XmlSerializer.Serialize (Namespaces TextWriter, TextWriter, Object, XmlSerializerNamespaces)
on System.Web.Services.Protocols.XmlReturnWriter.Write (response HttpResponse, Stream outputStream, Object returnValue)
on System.Web.Services.Protocols.WebServiceHandler.WriteReturns (Object[] returnValues)
on System.Web.Services.Protocols.WebServiceHandler.Invoke ()