getting error into encrypt and decrypt data using ASP.NET

I am using this code for Encrypt and Dycrpt the data using asp.net .

But i am getting the error:--

**Specified initialization vector (IV) does not match the block size for this algorithm**. 

here is a code:-

my .cs file is:-

public static class Encrypt_Decrypt
{
static Encrypt_Decrypt()
{

}
public static string EncryptString(string ClearText)
{

    byte[] clearTextBytes = Encoding.UTF8.GetBytes(ClearText);

    System.Security.Cryptography.SymmetricAlgorithm rijn = SymmetricAlgorithm.Create();

    MemoryStream ms = new MemoryStream();
    byte[] rgbIV = Encoding.ASCII.GetBytes("hanuservicestalknsolve");
    byte[] key = Encoding.ASCII.GetBytes("hanuservicestalknsolve");
CryptoStream cs = new CryptoStream(ms, rijn.CreateEncryptor(key, rgbIV),CryptoStreamMode.Write);

    cs.Write(clearTextBytes, 0, clearTextBytes.Length);

    cs.Close();

    return Convert.ToBase64String(ms.ToArray());
}
private static string DecryptString(string EncryptedText)
{
    byte[] encryptedTextBytes = Convert.FromBase64String(EncryptedText);

    MemoryStream ms = new MemoryStream();

    System.Security.Cryptography.SymmetricAlgorithm rijn = SymmetricAlgorithm.Create();


    byte[] rgbIV = Encoding.ASCII.GetBytes("hanuservicestalknsolve");
    byte[] key = Encoding.ASCII.GetBytes("hanuservicestalknsolve");
    CryptoStream cs = new CryptoStream(ms, rijn.CreateDecryptor(key, rgbIV),
    CryptoStreamMode.Write);

    cs.Write(encryptedTextBytes, 0, encryptedTextBytes.Length);

    cs.Close();

    return Encoding.UTF8.GetString(ms.ToArray());

}
}

Here is my aspx.cs code:

 string eventi = Encrypt_Decrypt.EncryptString(DataBinder.Eval(e.Item.DataItem,     "name_of_post_id").ToString());
    string post = Encrypt_Decrypt.EncryptString(DataBinder.Eval(e.Item.DataItem, "compreq_eventid").ToString());