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?