Problem in variable substitution

Hi experts,

I need help for a strange problem:

I have a proxy to file scenario in which I create an xml file; I need to use a field of the mapping to create filename.

In Receiver CChannel I have used variable substitution, but in message monitoring I got this error:

Delivery of the message to the application using connection File_http://sap.com/xi/XI/System failed, due to: com.sap.aii.af.ra.ms.api.RecoverableException: Error during variable substitution: com.sap.aii.adapter.file.varsubst.VariableDataSourceException: The following variable was not found in the message payload: Folio: com.sap.aii.adapter.file.configuration.DynamicConfigurationException: Error during variable substitution: com.sap.aii.adapter.file.varsubst.VariableDataSourceException: The following variable was not found in the message payload: Folio.

FileName Scheme is: DM65_%Folio%.xml

In variable substitution I have used this reference:

Folio = payload:MT_MexicanElectronicInvoice_MyFirm,1,CABFAC,1,Folio,1

This is the payload of the message that ends in error:

<?xml version="1.0" encoding="UTF-8" ?>

- <ns0:MT_MexicanElectronicInvoice_MyFirm xmlns:ns0="urn:MyFirm-EDICOM:MexElectronicInvoice">

- <CABFAC>

<IdLabel>CABFAC</IdLabel>

<Version>3.0</Version>

<Folio>2100000003</Folio>

<FecHac>16-11-2011 17:11:17</FecHac>

<FPago>Advance payment</FPago>

<Subtotal>9.976,00</Subtotal>

<TotalFac>11.568,16</TotalFac>

......

What is the problem? wrong reference?

thanks

Fabio