Workflow Rules - How to check if invoice Created By User is active?

Summary:

We have a requirement to route invoices for approval based on the Created By user and send to their manager. This works fine when the Created By user is an active employee. However, when they have been terminated the Workflow errors out because it cannot find an active supervisor, since there is no active assignment record for the Created By user.

How can we check if the Created By user is active? If they are active, we would route to manager, if they are inactive we would route differently.

Content (required):

We have a requirement to route invoices for approval based on the Created By user and send to their manager. This works fine when the Created By user is an active employee. However, when they have been terminated the Workflow errors out because it cannot find an active supervisor, since there is no active assignment record for the Created By user.