Index was out of range

I am running dbContext.BulkCopyAsync twice. It works the first time. The second time it completes the insert, but then throws an index out of range error. If I continue, then I get a Npgsql.NpgsqlException: 'Exception while reading from stream' TimeoutException: Timeout during reading attempt. I am using EF Core 5.0. I have a converter in place to handle my ulongs. There are ulongs in the orgs type, so that doesn’t seem to be it.

Exception message: 
System.ArgumentOutOfRangeException: 'Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')'

Stack trace:
This exception was originally thrown at this call stack:
    System.ThrowHelper.ThrowArgumentOutOfRange_IndexException() in ThrowHelper.cs
    Npgsql.NpgsqlDataReader.ProcessMessage(Npgsql.IBackendMessage) in NpgsqlDataReader.cs
    Npgsql.NpgsqlDataReader.NextResult(bool, bool, System.Threading.CancellationToken) in NpgsqlDataReader.cs
    System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() in ExceptionDispatchInfo.cs
    System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task) in TaskAwaiter.cs
    System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task) in TaskAwaiter.cs
    Npgsql.NpgsqlCommand.ExecuteReader(System.Data.CommandBehavior, bool, System.Threading.CancellationToken) in NpgsqlCommand.cs
    System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() in ExceptionDispatchInfo.cs
    System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task) in TaskAwaiter.cs
    System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task) in TaskAwaiter.cs
    ...
    [Call Stack Truncated]

Steps to reproduce

var options = new LinqToDB.Data.BulkCopyOptions
{
    KeepIdentity = true
};

await dbContext.BulkCopyAsync(options, orgs);
await dbContext.BulkCopyAsync(options, leagues); // index out of range

public class League
{
    public int Id { get; set; }
    public LeagueState State { get; set; }

    public ulong ChannelId { get; set; }

    public string Initials { get; set; } = string.Empty;

    public string? Image { get; set; }

    public int OrgId { get; set; }

    public Org Org { get; set; }

    public int Season { get; set; }

    public int? WarMatchId { get; set; }

    public List<Participant> Participants { get; set; }
}

Environment details

linq2db version: 3.1.6 Database Server: Postgres Database Provider: Npgsql Operating system: Windows 10 .NET Framework: .net 5.0

[MaceWindu]

Looks like something wrong with async in npgsql 5, as we are getting errors from async code even from other providers when run tests alongside with npgsql. I will investigate and report it to npgsql repo

[devhl-labs]

That worked though.