Postgres 7.3

I'm using the copy command to fill my tables. I've got this file with the data with delimiter '\t' and 'isnull' for nulls as follows:

copy mytable FROM '/usr/share/postgresql/data1/skripte/Daten/mytable' using delimiters '\t' with null as 'isnull';

But I receive an error saying that 'the value is too long for type varying(1)' This because I am filling a column defined as numeric(1) with nulls. Or in other cases I get a 'bad numeric input'

But if I insert some values into the table with 'insert', including nulls and then do

copy mytable TO '/usr/share/postgresql/data1/skripte/Daten/mytable' using delimiters '\t' with null as 'isnull';

the file looks exactly the same as my original one!!How do I insert nulls into a numeric field with the copy command??

Am i missing something??