This is the exception, I am seeing very first time in SharePoint development. What I tried was, through SharePoint designer I tried to update the list item. The source and destination lists are having a field of type choice. I want to grab the choice field information from source list and update the destination list choice field.
The problem occurs only when the source choice field is empty.
More clearly, I have a list with checkbox list field. When a new list item is added, the workflow triggers and start updating the another list item with the current item information. And when I select nothing in the checkbox list field, then it throws the exception "SharePoint Exception: Coercion Failed: Input cannot be null for this coercion".
So, why the problem comes?
Because in update list item phrase in SharePoint designer I have selected the lookup for choice Return field as as Choices, Comma delimited. See the below screenshot.
Hope this helps to understand it well and solve your problem.