\copy ... with null as '' csv doesn't get nulls

I'd expect this:

\copy tablename from 'filename.csv' WITH NULL as '' CSV HEADER

whould import "" as NULL.

The input file is UTF-8 (not Unicode/UTF-16).
I checked the hexdump and the "wannabe" NULL are actually
2c 22 22 2c -> ,"",

all fields are varchar that admit NULL

but still I get empty strings ('') in spite of NULL.

Am I missing something or it is a well known "feature"?

thx

--
Ivan Sergio Borgonovo
http://www.webthatworks.it