Registering at http://v2.wallabag.org/login, I just got exactly the same symptoms. See:
Seems fixed now, though the name appears as "wallabag¬" - not sure if the last character is deliberate? (Hint: even if it is, it looks like it isn't, which might not be what you want!)
Better to error if not set, let the dev explicitly put in some dummy values that's ever desired - that's why I said above "not sure why you're doing this", I assumed it was a setting of yours, not a '[neglect-to-]setting'!