Why would a User Event Script fail only on newly-created records?

We have a user-event script that runs upon submission (afterSubmit) of a Service Report. The script creates Time entries and Note records, saving them to the Project record associated with that Service Report. After editing a Service Report that already exists, the script runs without issue. However, when a new Service Report is created and then submitted the script throws an error.

That error says "Invalid API usage. You must use getValue to return the value set with setValue."

I've looked over the script and made sure all my calls to getValue and getText occur before any calls to setValue, so I don't understand where the error originates. The stack trace ends at the afterSubmit function, but all my API calls reside in other functions I've defined. The line number given is the first line of the afterSubmit function, where I'm passing scriptContext.newRecord to one of those other functions. All that function does is extract the data I need from the Service Report and return it in an object. It does no setting at all.