Specified key is not a valid size for this algorithm.

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C#3.5

I use this code to encrypt a file but when i input the key (example :12345678) and building an error "Specified key is not a valid size for this algorithm."
This is my code:

public static void EncryptFile(string sInputFilename, string sOutputFilename, string sKey)
        {
            FileStream fsInput = new FileStream(sInputFilename,FileMode.Open,FileAccess.Read);
            FileStream fsEncrypted = new FileStream                     (sOutputFilename,FileMode.Create,FileAccess.Write);
            DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
            DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
            DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
            ICryptoTransform desencrypt = DES.CreateEncryptor();
            CryptoStream cryptostream = new CryptoStream(fsEncrypted,desencrypt,CryptoStreamMode.Write);
            byte[] bytearrayinput = new byte[fsInput.Length - 1];
            fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
            cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
        }

How to solve this problem?
Sorry if my english is not good.