ASP.NET Async lambda expression converted to a 'Task'

The following code does not get compiled and unfortunately I could not figure out why.

using System.Text;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
app.MapGet("/", () => "Hello World!");
app.MapPost("/multi", async (ctx) =>
{
    Console.WriteLine("HTTP headers");
    Console.WriteLine(ctx.Request.Headers.ContentType);
    Console.WriteLine("??????????????????????????????????????");
    Console.WriteLine("HTTP body");
    string? line;
    ctx.Response.StatusCode = StatusCodes.Status201Created;
    var ms = new MemoryStream();
    await ctx.Request.Body.CopyToAsync(ms);
    using (StreamReader stream = new StreamReader(ms))
    {
        await stream.ReadToEndAsync()
        .ContinueWith(async t =>
        {
            var b = await t;
            Console.WriteLine(b);
        });
    }
    Console.WriteLine("??????????????????????????????????????");
    Console.WriteLine("FORM data");
    foreach (var form in ctx.Request.Form)
    {

        Console.WriteLine("Key: {0} => Value: {1}", form.Key, form.Value);
    }


    return await ctx.Response.WriteAsync("Hello"); 
});
app.Run("http://localhost:8080");

The compiler complains:

[{
    "resource": "/c:/5_.netcore/multipart/Program.cs",
    "owner": "msCompile",
    "code": "CS8031",
    "severity": 8,
    "message": "Async lambda expression converted to a 'Task' returning delegate cannot return a value. Did you intend to return 'Task<T>'? [C:\\5_.netcore",
    "startLineNumber": 34,
    "startColumn": 5,
    "endLineNumber": 34,
    "endColumn": 5
}]

What am I doing wrong?