Explicit NULL for no INTEGER data?

   I have a data file I want to copy into a table. The column name and
attribute is:

     depth_seal INTEGER

one of the rows in the .csv file I want to bring into the table has no value
for this column, but there is no explicit NULL in the text file. When I try
to copy the file psql complains:

     ERROR:  invalid input syntax for integer: ""
     CONTEXT:  COPY water_well, line 1, column depth_seal: ""

Do I need an explicit NULL in that column?

Rich