Getting "Invalid data" while DES-decrypting

Hello people,
I'm trying a simple encryption procedure, encrypting/decrypting a HEX value.
Encryption seems to work fine: result of encryption is "85772B6784BC1505".
While trying to decrypt I get an "Invalid data"-exception.
Does anybody have any idea what I'm doing wrong?
thanks much.
amine.
CODE:
        Dim x1 As UInt64 = DES.DES_EncryptCode("&H" & "0123456789ABCDEF")
        Dim x2 As UInt64 = DES.DES_DecryptCode(x1)
        Private Shared KEY_64() As Byte = {7, 6, 5, 4, 3, 2, 1, 0}
        Private Shared IV_64() As Byte = {0, 1, 2, 3, 4, 5, 6, 7}

        Public Shared Function DES_EncryptCode(ByVal code As UInt64) As UInt64
            Dim des As New System ...