F# Base64 encode and strict decode

open System
open System.Text

let encode (text: string) =
    text |> Encoding.UTF8.GetBytes |> Convert.ToBase64String

let tryDecode (encoded: string) =
    try
        let bytes = Convert.FromBase64String encoded
        let text = Encoding.UTF8.GetString bytes
        // round-trip check rejects malformed UTF-8 payloads
        if Encoding.UTF8.GetByteCount text = bytes.Length then Ok text
        else Error "payload is not valid UTF-8"
    with :? FormatException -> Error "payload is not valid Base64"