F# async does not support .NET TPL async

tpl_async_problem.fs

open System
open System.Net

let download(url : string) =
    async {
        let client = new WebClient()
        let! html = client.DownloadStringTaskAsync(url)
        return html 
    }

[<EntryPoint>]
let main argv = 
    let html = 
        download("http://example.com")
        |> Async.RunSynchronously
    0


//   This expression was expected to have type Async<'a> but here has type
  

See https://github.com/fsharp/fsharpx/blob/master/tests/FSharpx.Tests/TaskTests.fs