Cosmos DB update fails Unable to cast Guid to string

Tried to update a Cosmos DB record in ASP.NET core 3.1. But the update fails with the following message: "Unable to cast object of type 'System.Func'2[Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry,System.Guid]' to type 'System.Func'2[Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry,System.String]'

The error occurs on the saveCangesAsync(); Simplified, the code looks like:

// The service
public async Task<Todo> UpdateAsync(Todo entity)
{
  var response = ctx.Todos.Update(entity);
  await ctx.SaveChangesAsync(); // Error here
  return response.Entity;
}

// The entity Todo
public class Todo
{
  public Guid id { get; set; }
  [Required(ErrorMessage = "Description is required")]
  public string description { get; set; }
  ...
}

// The context
public class TodoDbContext : DbContext
{
    public DbSet<Todo> Todos { get; set; }
    public TodoDbContext(DbContextOptions<TodoDbContext> options) : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultContainer("Todos");
    }
}

// The controller
[HttpPut]
public async Task<IActionResult> Put(Todo todo)
{
    try
    {
        if (ModelState.IsValid)
        {
            Todo td = await service.GetAsync(todo.id.ToString());

            if (td != null)
            {
                td.description = todo.description;
                var response = await service.UpdateAsync(td);
                return Ok(response);
            }

            return BadRequest("Not found.");
        }
        else
        {
            return BadRequest(ModelState);
        }
    }
    catch (Exception ex)
    {
        return BadRequest(ex.Message);  // Exception here
    }
}

I can insert, read, but not update, so following code runs fine (with a Guid as well)

public async Task<Todo> CreateAsync(Todo entity)
{
    entity.id = Guid.NewGuid();
    var response = await ctx.Todos.AddAsync(entity);
    await ctx.SaveChangesAsync();
    return response.Entity;
}

Thanks for any help!