Coercion Failed: Input cannot be null for this coercion.

I created workflow in Nintex Workflow 2013 that contains a “For each” action that loops through a collection and performs several actions.  The workflow ran to a certain point and began throwing the error below and then stopped progressing.

Coercion Failed: Input cannot be null for this coercion.

After lots of trial and error, I finally figured out how to resolve the issue.  It turned out that immediately after the last successful action in the workflow that completed, the workflow had several “Set variable” actions that were disabled.  These “Set variable” actions were configured to perform a list lookup on another list.

I deleted these immediate “Set variable” actions (four in total) and republished the workflow.  The next time I triggered the workflow, the workflow successfully completed without encountering the error.