public class Bill
    {
        public int Id { set; get; }
        public string Name { set; get; }

        public virtual ICollection<Transaction> Transactions { set; get; }
    }

    public class Transaction
    {
        public int Id { set; get; }
        public DateTime AddDate { set; get; }
        public int Amount { set; get; }

        [ForeignKey("BillId")]
        public virtual Bill Bill { set; get; }
        public int BillId { set; get; }
    }

    public class MyContext : DbContext
    {
        public DbSet<Bill> Bills { get; set; }
        public DbSet<Transaction> Transactions { get; set; }
    }

    public class Configuration : DbMigrationsConfiguration<MyContext>
    {
        public Configuration()
        {
            AutomaticMigrationsEnabled = true;
            AutomaticMigrationDataLossAllowed = true;
        }

        protected override void Seed(MyContext context)
        {
            var bill1 = new Bill { Name = "bill-1" };
            context.Bills.Add(bill1);

            for (int i = 0; i < 11; i++)
            {
                context.Transactions.Add(new Transaction
                {
                    AddDate = DateTime.Now.AddDays(-i),
                    Amount = 1000000000 + i,
                    Bill = bill1
                });
            }
            base.Seed(context);
        }
    }

    public static class Test
    {
        public static void RunTests()
        {
            Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyContext, Configuration>());
            using (var context = new MyContext())
            {
                var sum = context.Transactions.Sum(x => x.Amount);
                Console.WriteLine(sum);
            }
        }
    }

SELECT 
         [GroupBy1].[A1] AS [C1]
         FROM ( SELECT 
                    SUM([Extent1].[Amount]) AS [A1]
                    FROM [dbo].[Transactions] AS [Extent1]
                    )  AS [GroupBy1]

 Arithmetic overflow error converting expression to data type int.

var sum2 = context.Transactions.Sum(x => (Int64)x.Amount);

SELECT 
      [GroupBy1].[A1] AS [C1]
         FROM ( SELECT 
                    SUM( CAST( [Extent1].[Amount] AS bigint)) AS [A1]
                    FROM [dbo].[Transactions] AS [Extent1]
               )  AS [GroupBy1]                  

var date = DateTime.Now.AddDays(10);
var sum3 = context.Transactions
                  .Where(x => x.AddDate > date)  
                  .Sum(x => (Int64)x.Amount);               

SELECT 
     [GroupBy1].[A1] AS [C1]
        FROM ( SELECT 
                    SUM( CAST( [Extent1].[Amount] AS bigint)) AS [A1]
                    FROM [dbo].[Transactions] AS [Extent1]
                    WHERE [Extent1].[AddDate] > @p__linq__0
              )  AS [GroupBy1]

The cast to value type 'Int64' failed because the materialized value is null.
Either the result type's generic parameter or the query must use a nullable type.

var date = DateTime.Now.AddDays(10);
var sum3 = context.Transactions
                  .Where(x => x.AddDate > date)
                  .Sum(x => (Int64?)x.Amount) ?? 0;

 var sum4 = context.Bills.First().Transactions.Sum(x => (Int64?)x.Amount) ?? 0;

SELECT 
     [Extent1].[Id] AS [Id], 
     [Extent1].[AddDate] AS [AddDate], 
     [Extent1].[Amount] AS [Amount], 
     [Extent1].[BillId] AS [BillId]
   FROM [dbo].[Transactions] AS [Extent1]
   WHERE [Extent1].[BillId] = @EntityKeyValue1

var entry = context.Bills.First();
var sum5 = context.Entry(entry).Collection(x => x.Transactions).Query().Sum(x => (Int64?)x.Amount) ?? 0;

SELECT 
    [GroupBy1].[A1] AS [C1]
     FROM ( SELECT 
               SUM( CAST( [Extent1].[Amount] AS bigint)) AS [A1]
                   FROM [dbo].[Transactions] AS [Extent1]
                    WHERE [Extent1].[BillId] = @EntityKeyValue1
            )  AS [GroupBy1]