Private Key Encryption with BouncyCastle

I am using the version of BouncyCastle.NET and I have to keep the RSA private key for the file explicitly encrypted with a password for security reasons.

What I'm trying to do now is this:

Dim rand As New SecureRandom
    Dim arr As Byte() = New Byte(7) {}
    rand.NextBytes(arr)

    Dim privateKeyInfo As EncryptedPrivateKeyInfo = EncryptedPrivateKeyInfoFactory.CreateEncryptedPrivateKeyInfo(
"PBEwithHmacSHA-256",
 Repository.Password.ToCharArray,
 arr,
 1,
 data.BouncyCastlePrivateKey
)

But BouncyCastle raises a NullReferenceException in the last statement. Since the method is completely undocumented> :( I wonder if any of you know how to use it correctly ...

(none of my NULL parameters, by the way, already checked that)