Getting "NOT NULL constraint violated" for a column with a DEFAULT value

I have a column defined as follows:

"div5tailnum" VARCHAR(10) NOT NULL DEFAULT ''

(that's the output of \d mytable), or if you like:

Div5TailNum VARCHAR(10) NOT NULL DEFAULT ''

(that's from the CREATE TABLE command). I'm trying to load data from a CSV file using the following SQL:

COPY OFFSET 2 INTO mytable FROM file.csv USING DELIMITERS ',','\n','"' NULL AS '';

but I get:

INSERT INTO: NOT NULL constraint violated for column ontime.div5tailnum

shouldn't that be impossible, seeing how I've provided a default?