Concurrency problem with no parallelism

gistfile1.txt

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var sch = new ConcurrentExclusiveSchedulerPair().ExclusiveScheduler;
            var tf = new TaskFactory(sch);

            var task = tf.StartNew(Main).Result;
            task.Wait();
        }

        static async Task Main()
        {
            var t1 = NonThreadSafe();
            var t2 = NonThreadSafe();

            await t1;
            await t2;
        }

        static int SomeNumber = 0;

        static async Task NonThreadSafe()
        {
            var sn = SomeNumber;
            SomeNumber += 1;

            await Task.Yield();
            Debug.Assert(SomeNumber == sn + 1);
        }
    }
}