cannot insert explicit value for identity column in table '' when identity_insert is set to off

I am getting the error
{"could not insert: [OfficeSuite.AP.Entity.TravelInvoice#2573][SQL: INSERT INTO tbl_ap_travelinvoice (IsPushedToTD, ID) VALUES (?, ?)]"}
Cannot insert explicit value for identity column in table 'tbl_ap_travelinvoice' when IDENTITY_INSERT is set to OFF.

The above error is happeneing for the tables where i am using with JoinedSubClass . all othere tables are working fine.

below is my code for mapping the table

public class APInvoiceMap : ClassMap<APInvoice>
{
public APInvoiceMap()
{
ImportType<APInvoiceItemLookUp>();
Table("tbl_ap_invoice");
Id(x => x.Id).Column("ID").GeneratedBy.Native();

References(x => x.ItemStatus).Column("StatusCode").Cascade.None();

.KeyColumn("InvoiceID").Cascade.AllDeleteOrphan().Table("tbl_ap_invoiceitem").Inverse().Not.LazyLoad()
;
HasMany(x => x.InvoiceServiceMappings)
.KeyColumn("InvoiceID").Cascade.AllDeleteOrphan().Table("tbl_ap_invoiceservicemapping").Inverse().Not.LazyLoad()
;
HasMany(x => x.APCreditNoteInvoiceItems)
.KeyColumn("InvoiceID").Cascade.All().Table("tbl_ap_creditnoteinvoiceitem").Inverse().Not.LazyLoad()
;

JoinedSubClass<TravelInvoice>("ID", m =>
{
m.Table("tbl_ap_travelinvoice");

m.Map(x => x.IsPushedToTD).Column("IsPushedToTD");

m.HasMany(x => x.TravelInvoiceItemList)
.KeyColumn("InvoiceID").Cascade
.AllDeleteOrphan().Table("tbl_ap_travelinvoiceitem")
.Inverse().LazyLoad();
m.HasMany(x => x.TravelRequestItemList)
.KeyColumn("TravelInvoiceID").Cascade.AllDeleteOrphan().Table("tbl_ap_travelrequestitem").Inverse().LazyLoad();

});

}
}