COPY with null

I have seen this question asked, but the answer doesn't seem to have worked for the other person or myself. I have several files I need to copy into postgres and if I try
COPY compmstr from '/user/local/testfile' using delimiters '\t' with null as '' (or '\N' or '\t\t' or anything else);
I get ERROR: copy: line 1, Bad numeric input format '' (or whatever I used). I am using files that can have numeric nulls and/or character nulls

Anyone figure this one out? I'm on 7.3.2

Karen Smallwood
Programmer/Analyst
Mighty Distributing System of America, Inc.
650 Engineering Drive
Norcross, Georgia 30092
(770) 448-3900
(770) 446-8627 fax