If you experience the following error during the delivery execution for a recipient:
This happens when you use a target mapping in a delivery which is different than the incoming data in the delivery activity of a workflow. So suppose you queried recipeint table and passed it to a delivery, and in the delivery, you have a custom targetMapping, you will get this error.
Secondly, if you are using a custom target mapping in a delivery and it has any personalization block or any column referencing in its content, and you use the recipient object, you will get this error. e.g. if you call recipient.id or recipient.email