AttributeError: 'NoneType' object has no attribute 'fieldtype' when trying out a print format without "naked"

When editing a print format, I used to be able to hit the print icon to see if my code is working. That is, without an actual document loaded, straight from the “Custom Format” page.

Now with version 14 (or maybe completely unrelated to that, I am not sure) I keep getting the well known

AttributeError: 'NoneType' object has no attribute 'fieldtype'

Printing an actual document is fine but is a little tiring as a test cycle. Am I doing things the wrong way?

Cheers,
kombi