Well, what about None?

$ python3 -c 'import csv; reader = csv.reader("foo", delimiter=None)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: delimiter must be set

English grammatically speaking, we should get this kind of error:
ValueError: "delimiter" must be string, not None

But computer science-ly speaking, the exception message "delimiter must be set" is correct because setting null or None value to a variable can be considered as same as unsetting that variable, hence error message "must be set".

And I would argue the empty string can be considered as one of a kind with None in this case.

But we'll see other people's opinions. I am also not sure about this case.