Nil in CSV Fixtures


The Guide to Testing the Rails
says about CSV fixtures:

nulls can be achived by just placing a comma, for example,
(1,sclaus,false,) minus the parenthesis of course.

I tried that, but after the fixtures had been created, I checked the
value of the field in the DB and instead of nil the value was zero,
eventhough the default value for the field is NULL.
Is the guide wrong?