Unable to sign message from the generated key pair

[MdTeach]

I've generated the pgp pairs as:

package(url: "https://github.com/krzyzanowskim/ObjectivePGP.git", from: "0.99.4")

let key = KeyGenerator(
   algorithm: .RSA, keyBitsLength: 2048, cipherAlgorithm: .AES128, hashAlgorithm: .SHA256
).generate(for: "",passphrase:"")

let publicKey = try key.export(keyType: .public)
let privateKey = try key.export(keyType: .secret)

But when I try to sign the message

func useEmptyPassPhrase(key: Key?) -> String? { return "" }

let message = "Some Message"
let messageData = message.data(using: .utf8)!
let signature = try ObjectivePGP.sign(
      messageData, detached: true, using: [privateKey], passphraseForKey: useEmptyPassPhrase)

But I'm getting following error:

failed: caught error: "Error Domain=com.objectivepgp Code=-1 "Unable to sign. Can't create any signature for keys." UserInfo={NSLocalizedDescription=Unable to sign. Can't create any signature for keys.}"

Is there something I'm missing during the key generation ?? As I also tried importing the pgp private key generated from https://www.npmjs.com/package/openpgp and it works with ObjectivePGP.sign