[2005] Error : Specified Key Is Not A Valid Size For This Algorithm

I have found one sample for encrypting/decrypting text using System.Security.Cryptography library (Encrypt/Decrypt string VB.Net).
But there is an error when I try to compile it:

Quote:

Specified key is not a valid size for this algorithm

Where is an error? And what key I should use? Sample:

Imports System.Security.Cryptography
Private Shared Function Encrypt(ByVal strText As String, ByVal strEncrKey As String) As String
Dim IV() As Byte = {&H12, &H34, &H56, &H78, &H90, &HAB, &HCD, &HEF}

[code]....