Background:

There is a SharePoint 2013 list workflow to send email on list item created. To and Cc fields for this email are set based on some If conditions. Email body has form fields for quick reference.

Issue:

Whenever a new list item is created, workflow gets Cancelled and displays below error in Workflow History list:


"Coercion Failed: Input cannot be null for this coercion."



Cause:

After checking workflow multiple times, for each of its fields like To, Cc and form fields inside email body, I found that there was a multiple select field which is set to return as "Choices, Comma Delimited" value. When this value is null, then we get above error.

Solution:

As mentioned above the cause of issue, changed return type for problematic field from "Choices, Comma Delimited" to "As String". This solved the issue and workflow runs without any errors.

Hope it helps someone.