copy from file with null date fields

I'm running Postgres 6.3.2 on a Sparc20 running Solaris 2.5.1 and I'm
having difficulty copying data from certain text files where fields
expected to be type 'date' are null.  When the copy encounters the
null field it stops and prints the message:

ERROR:  Bad date external representation

I have purposely _not_ defined the field as 'not null'.  I have not
had trouble reading from files where other types of fields are null.

Any ideas, or is this a known bug?

--
William D. McCoy
Geosciences
University of Massachusetts
Amherst, MA  01003