Can't connect via private key.

I'm trying to connect to a server via private key.

From the CLI, ssh -i ./gen.pem ubuntu@<ip> works as intended, so I know the key, username, port and IP are correct.

So far, this is the code:

let tcp = TcpStream::connect((ip, 22)).unwrap();
let mut sess = Session::new().unwrap();
sess.set_tcp_stream(tcp);
sess.handshake().unwrap();
sess.userauth_pubkey_file("ubuntu", None, Path::new("./gen.pem"), None).unwrap();

But the last line of code fails with:

thread 'main' panicked at 'called Result::unwrap() on an Err value: Error { code: Session(-18), msg: "Username/PublicKey combination invalid" }', src/main.rs:138:10

The same username and private key works with SSH.

Start with the userauth_pubkey_file call in the reported src/main.rs example and inspect how this authentication path handles the supplied key. Compare its behavior with the working SSH command and determine what evidence would distinguish a key-format, username, or library-authentication problem; done means reproducing the failure and documenting or testing a concrete cause.