NHibernate Error Message: Invalid index 3 for this SqlParameterCollection with Count=3

I have a test database design like this:

The following is the pseudo-code:

//BhillHeader
public class BillHeader
{
    public BillHeader()
    {
        BillDetails = new List<BillDetail>();
    }
    public virtual int BillNo { get; set; }
    public virtual IList<BillDetail> BillDetails { get; set; }
    public virtual decimal Amount { get; set; }

    public virtual void AddDetail(BillDetail billdet)
    {
        BillDetails.Add(billdet);
    }
}  

//BillHeader Map
public class BillHeaderMap : ClassMap<BillHeader>
{

    public BillHeaderMap()
    {
        Table("BillHeader");
        LazyLoad();
        Id(x => x.BillNo).GeneratedBy.Identity().Column("BillNo");
        Map(x => x.Amount).Column("Amount").Not.Nullable();
        HasMany(x => x.BillDetails).KeyColumn("BillNo").Cascade.All().Inverse();
    }
}  

//BillDetail
public class BillDetail
{
    public BillDetail() { }
    public virtual int BillID { get; set; }
    public virtual int SeqNo { get; set; }
    public virtual BillHeader BillHeader { get; set; }
    public virtual decimal Amt { get; set; }

    public override bool Equals(object obj)
    {
        var other = obj as BillDetail;

        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;

        return this.BillID == other.BillID &&
            this.SeqNo == other.SeqNo;
    }

    public override int GetHashCode()
    {
        unchecked {
            int hash = GetType().GetHashCode();
            hash = (hash * 31) ^ SeqNo.GetHashCode();
            hash = (hash * 31) ^ BillID.GetHashCode();

            return hash;
        }
    }
}


//BillDetail Map
public class BillDetailMap : ClassMap<BillDetail>
{

    public BillDetailMap()
    {
        Table("BillDetail");
        LazyLoad();
        CompositeId().KeyProperty(x => x.BillID, "BillNo").KeyProperty(x => x.SeqNo, "SeqNo");
        References(x => x.BillHeader).Column("BillNo");
        Map(x => x.Amt).Column("Amt").Not.Nullable();
    }
}


//-----------------------------------------------------------------------------------------------------------------------------

//Program
public createBillNo()
{
    var sessionFactory = CreateSessionFactory();
    using (var session = sessionFactory.OpenSession()) {
        using (var sqlTrans = session.BeginTransaction()) {

            BillHeader billNo1 = new BillHeader() { Amount = 2500.00M};
            BillDetail bh11 = new BillDetail() { SeqNo = 1, Amt = 200.00M };
            BillDetail bh12 = new BillDetail() { SeqNo = 2, Amt = 300.00M };
            BillDetail bh13 = new BillDetail() { SeqNo = 3, Amt = 500.00M };

            AddBillDetailsToBillHeader(billNo1, bh11, bh12, bh13);
            session.SaveOrUpdate(billNo1); 
            sqlTrans.Commit();
        }
    }
}

private void AddBillDetailsToBillHeader(BillHeader billHeader, params BillDetail[] billDetails)
{
    foreach (var billdet in billDetails) {
        billHeader.AddDetail(billdet);
        billdet.BillHeader = billHeader;
    }
}

When I run this I'm getting the following exception:

Invalid index 3 for this SqlParameterCollection with Count=3

Please help me to resolve this issue.