Bouncy Castle Smart Card Certificate

Question

I work on win 7 professional and c#(VS2010).I need to sign a file and create a PKCS envelope.
The certificate to use for sign is on a USB smart card (I installed the crypto service provider CSP provided with the smart card).
This is the code that I wrote using bouncy Castle

private byte[] SignData(byte[] DataTosign)
{
    X509Certificate2 card = GetCertificate();
    Org.BouncyCastle.X509.X509Certificate cert = DotNetUtilities.FromX509Certificate(card);

    Org.BouncyCastle.Crypto.AsymmetricKeyParameter K = DotNetUtilities.GetKeyPair(card.PrivateKey).Private;

    CmsSignedDataGenerator generator = new CmsSignedDataGenerator();
    generator.AddSigner(K, cert, CmsSignedDataGenerator.EncryptionRsa, CmsSignedDataGenerator.DigestSha1);

    ArrayList certList = new ArrayList();
    certList.Add(cert);

    Org.BouncyCastle.X509.Store.X509CollectionStoreParameters PP = new Org.BouncyCastle.X509.Store.X509CollectionStoreParameters(certList);
    Org.BouncyCastle.X509.Store.IX509Store st1 = Org.BouncyCastle.X509.Store.X509StoreFactory.Create("CERTIFICATE/COLLECTION", PP);

    generator.AddCertificates(st1);

    CmsSignedData signedData = generator.Generate("1.2.840.113549.1.7.1", new CmsProcessableByteArray(DataTosign), true);  // <- here is the flag

    return signedData.GetEncoded();
}


public static X509Certificate2 GetCertificate()
{
    X509Store st = new X509Store(StoreName.My, StoreLocation.CurrentUser);
    st.Open(OpenFlags.ReadOnly);
    X509Certificate2Collection col = st.Certificates;
    X509Certificate2 card = null;
    X509Certificate2Collection sel = X509Certificate2UI.SelectFromCollection(col, "Certificates", "Select one to sign", X509SelectionFlag.SingleSelection);
    if (sel.Count > 0)
    {
        X509Certificate2Enumerator en = sel.GetEnumerator();
        en.MoveNext();
        card = en.Current;
    }
    st.Close();
    return card;
}

The problem is in Org.BouncyCastle.Crypto.AsymmetricKeyParameter K = DotNetUtilities.GetKeyPair(card.PrivateKey).Private; where I get an

System.Security.Cryptography.CryptographicException(Message Specified type is not valid)

It seems the problem is that the privatekey is on the smart card, but to use generator.AddSigner, I must pass the privatekey. Can someone help me on how to sign with a certificate on a smart card?

Many thanks!