For Loop Container, Script Task: An Error Message

I everyone,
I'm learning SSIS from a book but the most simple example is not working.
The "For Loop task" I assigned "xyz" as name.
I added a variable onto the "variables" window and called it "counter" Its scope was set to "xyz".
Inside the "for loop" ("xyz") editor i assigned values as follows:
"initexpression"-"@counter=0"
"eval expression"-"@counter<5"
"assign expression"-"@counter=@counter+1"

I added "Script task" onto the "For each" container and into the "Script task editor" I assigned
the following values:
"ReadOnlyVariables"-"counter"
Its design script is set to:[code]
Imports System
Imports System.Data
Imports System.Math
Imports Microsoft.SqlServer.Dts.Runtime

Public Class ScriptMain
Public Sub Main()
Dim variables As Variables
If Dts.Variables.Contains("Counter") = True Then
Dts.VariableDispenser.LockOneForRead("Counter", variables)
End If

MsgBox("You are in iteration: " & CStr(variables("Counter").Value))
Dts.TaskResult = Dts.Results.Success

End Sub
End Class
[/code]
Runnig it i get the following error message:









DTS Script task has encountered an exeption in user code:
Project namecriptTask_e7d98dbad0de4041bcdc9079a5c2fa65
Object reference not set to an instance of an object..
The line where the error occurs is from within the above script:
[code]
"MsgBox("You are in iteration:" " & CStr(variables("Counter").value))[/code]
Anyone understands what that means? What is "object reference" and how do i set it to an instance of an object?
Thanks