BPM workflow issue with Invoking BO

I have a workflow designed in a post processing method directive that starts out by running the GetList on the Project dataset, invokes the GetByID, and the Update Method to update a UD field in the Project table. It actually works well except for the beginning of the workflow.
Taking the list of ProjectIDs pulled by the GetList and then feeding them one at a time to the GetByID method to process the update has been elusive for me. If I just enter a project ID individually it works fine.
What would be the best way to take a list of the project IDs, and feed them through the workflow?

Thanks
Jason