Error occured :Invalid length for a Base-64 char array. in the following descrypt function

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public string Decrypt(string str)
{
str = str.Replace(" ", "+");
string DecryptKey = "2013;[pnuLIT)WebCodeExpert";
byte[] byKey = { };
byte[] IV = { 18, 52, 86, 120, 144, 171, 205, 239 };
byte[] inputByteArray = new byte[str.Length];

byKey = System.Text.Encoding.UTF8.GetBytes(DecryptKey.Substring(0, 8));
DESCryptoServiceProvider des = new DESCryptoServiceProvider();
inputByteArray = Convert.FromBase64String(str.Replace(" ", "+"));
MemoryStream ms = new MemoryStream();
CryptoStream cs = new CryptoStream(ms, des.CreateDecryptor(byKey, IV), CryptoStreamMode.Write);
cs.Write(inputByteArray, 0, inputByteArray.Length);
cs.FlushFinalBlock();
System.Text.Encoding encoding = System.Text.Encoding.UTF8;
return encoding.GetString(ms.ToArray());
}

how to solve the above error Please help.........