help with passkey authentication

I'm trying to implement passkey authenticator on iOS.

while register works perfectly I'm still struggling with authenticating.

let's say this is the code I'm using to authenticate:

override func provideCredentialWithoutUserInteraction(for credentialRequest: ASCredentialRequest){

    guard let req: ASPasskeyCredentialRequest = credentialRequest as? ASPasskeyCredentialRequest else { return }

    let hashedRp = hashRP(req.credentialIdentity.serviceIdentifier.identifier)

    do {
        let privateKey: P256.Signing.PrivateKey = try P256.Signing.PrivateKey(derRepresentation: Data(base64Encoded: CredentialProviderViewController.base64PrivateString) ?? Data([]))

        let ad = hashedRp + [29, 0, 0, 0, 0]
        let sig = try privateKey.signature(
            for: SHA256.hash(data: Data(ad + req.clientDataHash))
        )

        let res: ASPasskeyAssertionCredential = ASPasskeyAssertionCredential(
            userHandle: Data(hashedRp[0..<16]),
            relyingParty: req.credentialIdentity.serviceIdentifier.identifier,
            signature: sig.rawRepresentation,
            clientDataHash: req.clientDataHash,
            authenticatorData: Data(ad),
            credentialID: Data(hashedRp[0..<16])
        )

        extensionContext.completeAssertionRequest(using: res)

    } catch {}
}

this will produce:

authentication failed: 1 validation error for authenticationCredential __root__ string argument should contain only ascii characters.

what am i doing wrong?