XML to EDI conversion along with Filename Variable referencing field

Hi all - I have a situation where I am doing an IDOC to edi along with variable substitution in the filename. I would like the IDOC number in the filename.  The way I found to get the IDOC number was going into the payload (if there is another way to get IDOC number to put in the filename I would use that)

I get the error: Message could not be forwarded to the JCA adapter. Reason: javax.resource.ResourceException: The variable substitution can not parse the message xml: Content is not allowed in prolog.

If I remove the variable substitution where I am attempting to get the idoc all works well and I get my output file, thus my assumption is that there is something wrong with my substitution.

I am assuming the XML I am parsing is prior to the edi conversion thus the payload is

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

<ns0:ASC810_003010 xmlns:ns0="urn:spiritaero.com:B2B:ANSIX12">

<S_ISA>

  ....

  <D_I12>000278191</D_I12>

My variable substitution is

variable name: idoc

reference: payload:ASC810_003010,1,S_ISA,1,D_I12,1

THoughts on how to fix my situation?

Regards,

Diane