COPY command issue

COPY command issue

I'm a novice postgres user and I'm using 9.2: I'm using: COPY tagdata FROM
'C:/Filter112595/QF112595_3.csv' WITH DELIMITER ',' CSV HEADER
FORCE_NOT_NULL;
to read data into the table I created. The data are real, integers and dates.
I get this error:
ERROR: invalid input syntax for type real: "NULL" CONTEXT: COPY tagdata,
line 2, column residual: "NULL"
Before using FORCE_NOT_NULL, I had NULL as '' but changed it because of
the different data types.
Can someone explain what's going on? thanks!