Workflow error (JoinFieldvalue) <-- New bug (severity 3) discovered

I've got a workflow using a JoinField expression to get a value from an attached contact onto the current opportunity. Here are the details:
Workflow on object: Opportunity
Trigger: Before modified record saved
(no rule conditions)

Action:

Update field on oppty record named "Referrer EID", a custom short text field
value: JoinFieldValue('<Contact>',[<ContactId>],'<stReferrer_EID_ITAG>')

Error:

"...value too long for field 'Ztext_35' (maximum size 40)"

They're both short text fields in the oppty and the contact, each size 40. (This is in our test pod.)

Any insight would be greatly appreciated.

Thanks!
Mike Combopiano