Mapster ProjectToType breaks when using GroupBy/Select

Question

I use Mapster Mapper to project from domain models to DTOs. That works perfectly, but now I had to use a GroupBy/Select and that breaks my code:

public class Document
{

    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid Id { get; set; }

    public string? LanguageId { get; set; }

    public string? Slug { get; set; }
    public string? Name { get; set; }

}

public class DocumentDto
{

    public Guid Id { get; set; }

    public string? LanguageId { get; set; }

    public string? Slug { get; set; }
    public string? Name { get; set; }

}

public async Task<IList<T>> GetDocuments<T>(string? slug, string? languageId)
{
    var iq = _dataContext.Documents
        .Where(x => slug == null || (slug != null && x.Slug == slug))

        if (languageId != null)
        {
            iq = iq.GroupBy(x => x.Slug)
            .Select(x => x
                .OrderByDescending(y => y.LanguageId == languageId)
                .ThenByDescending(y => y.LanguageId == "da")
                .ThenBy(y => y.Name)
            .First());
        }
        else
        {
            iq = iq.OrderBy(x => x.Name).ThenBy(x => x.LanguageId);
        }

    return await iq.AsNoTracking().ProjectToType<T>().ToListAsync();
}

var documents = await GetDocuments<DocumentDto>(null, "en");

An unhandled exception has occurred while executing the request. System.Collections.Generic.KeyNotFoundException: The given key 'EmptyProjectionMember' was not present in the dictionary.

I have no idea how to handle this, what it means and where to look. Please help.