Assert in SpillSequenceSpiller

AssertRepro.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.2</TargetFramework>
    <LangVersion>latest</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="2.2.6" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="2.2.6" />
  </ItemGroup>

</Project>

Program.cs

using Microsoft.EntityFrameworkCore;
using System;
using System.Linq;
using System.Threading.Tasks;

namespace AssertRepro
{
    class Program
    {
        static async Task Main(string[] args)
        {
            using (var context = new ClientContext())
            {
                var matches = await context.BillingGroups
                .Select(bg => new MatchDto()
                {
                    BillingGroup = bg
                })
                .ToListAsync();                
            }
        }

        public class BillingGroup
        {
            public int Id { get; set; }
        }

        public class MatchDto
        {
            public BillingGroup BillingGroup { get; set; }
        }

        class ClientContext : DbContext
        {
            protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
            {
                optionsBuilder.UseInMemoryDatabase(Guid.NewGuid().ToString());
            }

            protected override void OnModelCreating(ModelBuilder builder)
            {
                base.OnModelCreating(builder);
            }

            public DbSet<BillingGroup> BillingGroups { get; set; }
        }
    }
}