ECDH using Android KeyStore generated private key

I'm trying to implement ECDH in Android using a private generated by Android KeyStore Provider.

public byte[] ecdh(PublicKey otherPubKey) throws Exception {

    try {
        ECPublicKey ecPubKey = (ECPublicKey) otherPubKey;
        KeyAgreement keyAgreement = KeyAgreement.getInstance("ECDH");
        PrivateKey pk = (PrivateKey) LoadPrivateKey("Backend");
        keyAgreement.init(pk);
        keyAgreement.doPhase(ecPubKey, true);

        return (keyAgreement.generateSecret());
    }
    catch (Exception e)
    {
        Log.e("failure", e.toString());
        return null;
    }
}

However, this exception is catched in keyAgreement.init(pk) :

E/failure: java.security.InvalidKeyException: cannot identify EC private key: java.security.InvalidKeyException: no encoding for EC private key

I generated before successfully the "Backend" Public/Private key pair using:

public void GenerateNewKeyPair(String alias)
        throws Exception {

    if (!keyStore.containsAlias(alias)) {
        // use the Android keystore
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_EC, ANDROID_KEYSTORE);
        keyGen.initialize(
                new KeyGenParameterSpec.Builder(
                        alias,
                        KeyProperties.PURPOSE_SIGN | KeyProperties.PURPOSE_VERIFY | KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
                        .setAlgorithmParameterSpec(new ECGenParameterSpec("secp256r1"))
                        .setDigests(KeyProperties.DIGEST_SHA256,
                                KeyProperties.DIGEST_SHA384,
                                KeyProperties.DIGEST_SHA512)
                        .setRandomizedEncryptionRequired(true)
                        .build());
        // generates the keypair
        KeyPair keyPair = keyGen.generateKeyPair();
    }

}

And I load the private key using:

public PrivateKey LoadPrivateKey(String alias) throws Exception {
    PrivateKey key = (PrivateKey) keyStore.getKey(alias, null);
    return key;
}

Anyone has an idea what is happening and can help me to understand how to fix it? Thanks!