Error in Variable Substitution, SFTP Receiver adapter

Hi ,

I am working on Proxy --> PI --> File scenario. At receiver end, we are using SFTP adapter.

I am getting the below error while using variable substitution but it works fine with FTP adapter.

"Delivery of the message to the application using connection SFTP_http://sap.com/xi/XI/SFTP failed, due to: com.sap.engine.interfaces.messaging.api.exception.MessagingException: javax.resource.ResourceException: javax.resource.ResourceException: The variable substitution can not be parse the message xml: Content is not allowed in prolog."

please suggest me .

Thanks

Dhileep