SharePoint Exception: Coercion Failed: Input cannot be null for this coercion

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.

Once changed that back to the As String then everything worked perfect. But, didn't get the reason behind. When nothing selected, then it shouldn't do any operation. It should check for null and do nothing. May be a built-in problem or something interesting with this.

Hope this helps to understand it well and solve your problem.