migrating SSIS from 2005 to 2008

I have script task in a package. It works fine on SQL2005. It doesn't work on SQL 2008. It does not give error message, just can not find the file.This is the code:

Imports System
Imports System.Data
Imports Microsoft.SqlServer.Dts.Runtime

Public Class ScriptMain

      Public Sub Main()

        If System.IO.File.Exists(Dts.Variables("FilePath").Value.ToString()) Then
            Dts.Variables("FileExists").Value = True
        Else
            Dts.Variables("FileExists").Value = False
        End If
        Dts.TaskResult = Dts.Results.Success
      
End Sub

End Class
Any ideas what could be the problem ?

Thank you