BPE_ADAPTER error on async BPM

I am trying to build the Collect Time-Dependent scenario in BPM and I'm getting a BPE_ADAPTER error. This is our first attempt at using BPM. The error says Unknown_Message, "No object type found for this message" and is part of category XI Adapter. I am trying to receive an IDoc, map it to a file structure, and append the file structure to a list. After a set time period has elapsed, I want to FTP the collected data file to the destination. I deleted everything from the process except the receive and send steps and I still get the error. It appears to be happening in the Call Adapter step in the XI pipeline. Any idea what could be wrong?