SSIS Script Task Reading Variables In Vb.net DTS

I'm trying this code in a script task inside a foreach loop that gets a file in a foreach file enumerator and maps it to a variable FileName.

 In the script task I'm setting the ReadOnlyVariables to @[User::FileName]

:
Imports System.IO
Imports System
Imports System.Data
Imports System.Math
Imports Microsoft.SqlServer.Dts.Runtime


Public Class ScriptMain


    Public Sub Main()
        Dim fname As String = CType(Dts.Variables("FileName").Value,
String)
        File.Move(fname, fname + ".processed")
        Dts.TaskResult = Dts.Results.Success
    End Sub


End Class


But now getting this error:


Error: Failed to lock variable "\xxxxDataf1.csv" for read access
with error 0xC0010001 "The variable cannot be found. This occurs when
an attempt is made to retrieve a variable from the Variables collection
on a container during execution of the package, and the variable is not
there. The variable name may have changed or the variable is not being
created.".

 

As you can see the variable does appear to be resolving.