The cast to value type 'System.Decimal' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

 _investmentRepository.Table.Where(t => t.CustomerId == customerId && t.InvestmentStatus == tempStatus).Sum(t =>t.Amount);

查詢結果存在NULL值，在嘗試將它轉換成System.Int32時出了錯，解決方法如下

_investmentRepository.Table.Where(t => t.CustomerId == customerId && t.InvestmentStatus == tempStatus).Sum(t => (decimal?)t.Amount) ?? 0;