Null Response from Request to Built-In Business Operation

I am sending an HL7 message from a BPL transform operation to a business operation that uses EnsLib.HL7.Operation.FileOperation.  

The context is that our application queues up outbound data that the business service polls every minute.  The objects in the queue are sent to the business process to transform and then to the operation to send outbound.  My goal is to have the business operation inform the business process (by a response) that the message was sent and what the message identifier is.  The business process would then send this back to the business service where it would be saved to the application's queue and mark the object in the queue as sent.  

I'm using a <call> tag in the BPL to call to the outbound operation.  I'm successfully passing in the transformed message via a context variable and the message is sending (right now just to a local folder on my file system). It seems, however the response is null.

I have tried setting the <call>'s response in the BPL editor to EnsLib.HL7.Message, to Ens.Response, and Ens.StringResponse, but as you can see below the standard EnsLib.HL7.Operation.FileOperation is returning a NULL response.  

Can I create a response from this outbound service without creating a custom outbound service?  

Thanks!