Using Jira Cloud (premium) for an automation rule to update the card due date when the WorkDuration of the card changes.
WorkDuration is custom number field
Rule trigger is WorkDuration value changes. If WorkDuration is not empty, set Due date to the start date plus WorkDuration.
Audit log gives me the following, and Due date is set to None. What is missing in the smart value to set the Due date?? The rule results in setting due date to none.

Exact text for due date is: {{issue.start date.plusdays(WorkDuration).days}}
I'd rather have business days but using days because it's a shorter value.