I'm trying to connect to an Ubuntu server with my private key, but I can't get it working

let execution () =
    let connectionInfo = new ConnectionInfo(host, "root", new PrivateKeyAuthenticationMethod("./resources/keys/id_rsa"))
    use client = new SshClient(connectionInfo)
    do client.Connect()
    ()
    
do execution ()

The code is a F# script but I guess it's readable.
I tested the file to be reachable (looks like the exception if the path is wrong is the same) and I also tried using the key with ssh cli, succesfully. So the key should be good.

EDIT: the same error occurs using both the NuGet package and the current develop branch on GitHub
EDIT 2: i managed to get it working. This is the working code:

let execution () =
    let connectionInfo = new ConnectionInfo(host, "root", new PrivateKeyAuthenticationMethod("root", new PrivateKeyFile("./resources/keys/id_rsa")))
    use client = new SshClient(connectionInfo)
    do client.Connect()
    ()
    
do execution ()

I see it's a bit redundant (having to specify twice the username). Is there a better way, similar to the multi-factor example?