连接组合键时出现InvalidOperationException异常

我有一个LINQ查询，在调用ToList()时在运行时抛出InvalidOperationException，我试图找出一种绕过它的方法，如果可能的话，而不必将连接分解成单独的where子句。

下面示例中的问题行是3和4。Loan_id是不可空类型的decimal, ti_disb_id是不可空类型的short。当我将这些语句拆分为单独的语句时，调用ToList()时没有异常。然而，它似乎更有意义，所以如果有一个解决方案，我很乐意听到。:)

        var baseQuery = (from a in context.ti_disbursements
                         join b in context.disbursement_schedule
                             on new {a.loan_id, a.ti_disb_id} equals
                                new {b.loan_id, b.ti_disb_id} into ab
                             from b in ab.DefaultIfEmpty()
                         join c in context.business_associates
                             on a.ba_id equals c.ba_id into ac
                             from c in ac.DefaultIfEmpty()
                         join d in context.investor_loan
                             on b.loan_id equals d.loan_id into ad
                             from d in ad.DefaultIfEmpty()
                         join e in context.loan_information
                             on d.loan_id equals e.loan_id into ae
                             from e in ae.DefaultIfEmpty()
                         join f in context.loan_balances
                             on e.loan_id equals f.loan_id into af
                             from f in af.DefaultIfEmpty()
                         join g in context.ti_information
                             on e.loan_id equals g.loan_id into ag
                             from g in ag.DefaultIfEmpty()
                         select new
                         {
                             loan_id = a.loan_id,
                             ti_disb_id = a.ti_disb_id,
                             ti_freq_id = a.ti_freq_id,
                             ti_disbursements_stop_code = a.ti_stop_code,
                             tax_account_number = a.tax_account_number,
                             ti_disb_due_dt = b.ti_disb_due_dt,
                             ti_expire_dt = b.ti_expire_dt,
                             ti_disb_amt = b.ti_disb_amt,
                             schedule_id = b.schedule_id,
                             ba_name = c.ba_name,
                             inv_bank_cd = d.inv_bank_cd,
                             inv_cd = d.inv_cd,
                             inv_group_cd = d.inv_group_cd,
                             loan_name = e.loan_name,
                             prin_bal = f.prin_bal,
                             ti_bal = f.ti_bal,
                             ti_information_stop_code = g.ti_stop_cd,
                             non_escrowed_type = a.non_escrowed_type,
                             ba_type_id = a.ba_type_id,
                             bill_received_dt = a.bill_received_dt,
                         });

异常消息:

InvalidOperation异常未被用户代码处理

转换为值类型'Int16'失败，因为物化值为空。结果类型的泛型参数或查询必须使用可空类型。

提前感谢!